short string optimization

The `ShortString` can represent zero-terminated strings up to `MaxSize` chars (excluding the terminating zero) and store a value to determine the length of the contained string in the last character `str[LenPos]` by storing `MaxSize - length` there. If the string to store has the maximal length of `MaxSize` (excluding the terminating zero) then `str[LenPos]` will store `0` and therefore act as the string terminator as well. For getting the string length back from that value just use `MaxSize - str[LenPos]`.

This allows to store `11`-chars strings in 32-bit mode and `15`-chars strings in 64-bit mode inline (for `UTF8`-encoded strings).

Instead of replicating the functionality of `GetString()` and `GetStringLength()` in `StringEqual()` it now calls these methods instead.

Since the payload (the `Data` union) of the current implementation of `GenericValue` is `12 bytes` (32 bit) or `16 bytes` (64 bit) it could store `UTF8`-encoded strings up to `10` or `14` chars plus the `terminating zero` character plus the string length:
``` C++
    struct ShortString {
        enum { MaxSize = sizeof(GenericValue::String) / sizeof(Ch) - sizeof(unsigned char) };
        Ch str[MaxSize];
        unsigned char length;
    };  // at most as many bytes as "String" above => 12 bytes in 32-bit mode, 16 bytes in 64-bit mode

```

This is achieved by introducing additional `kInlineStrFlag` and `kShortStringFlag` flags. When setting a new string value in `SetStringRaw(s, alloc)` it is first checked if the string is short enough to fit into the `inline string buffer` and if so the given source string will be copied into the new `ShortString` target instead of allocating additional memory for it.

Failing comparisons between certain (Ui|I)nt64 numbers

Some 64-bit integers cannot be represented losslessly as a double.
Due to a typo in the operator==, the comparison has been performed
after a double conversion in too many cases.

Fix comparison operator ambiguities between Value/Document

Before applying the simplifications in ed282b81, the SFINAE check for the
GenericValue(bool) constructor has been broken in MSVC 2005. Add a static
assert as a safe-guard against future reappearance of this problem.

This finally fixes #113.

In order to match GenericValue and its derived classes for the
SFINAE-implementation of some of the operators/functions, this
meta-function matches all types equal to or derived from a given
class.  See std::is_base_of<B,D> available in C++11.

Define RAPIDJSON_HAS_CXX_TYPETRAITS to use the C++11 implementation.

Some (older) compilers have problems with compile-time constants.
This commit simplifies the implementation of the helper classes
in order to improve compiler support, especially be removing the
need of partial template specialisation.

No functional changes.

Prepare equalto_operator tests to test comparisons between
 * GenericValue and GenericDocument
 * GenericValue with different SourceAllocator types

Both combinations currently fail due to ambiguities with the
templated operators on several compilers.

Replace tmpname() by mkstemp() on non-MSVC compilers

Three new APIs are added for JSON object type.

Fix `RAPIDJSON_HAS_STDSTRING is not defined` compilation warning

Fixes #118

Use local typedef to invoke GenericMember's destructor

As reported in #113, recent versions of Clang complain about
ambiguous overloads for some comparison operator instantiations,
especially if the deduced template type is a GenericValue.

Add an explicit, non-templated version for now (which is a better
match).

This only solves part of the problem, as comparisons between
  * GenericValue & GenericDocument
  * GenericValue with different SourceAllocator types
will still cause ambiguities.

GenericMemberIterator: fix comparisons/differences for mixed-constness expressions

Parsing optimization

Do unaligned non-SIMD matching first, and then SIMD matching later.
Also add a fast path for no skipping.

GenericValue: add optional support for std::string