This commit adds an IsGenericValue meta function to match arbitrary
instantiations of the GenericValue template (or derived classes).

This meta function is used in the SFINAE-checks to avoid matching
the generic APIs (operator=,==,!=; AddMember, PushBack) for instances
of the main template.  This avoids ambiguities with the GenericValue
overloads.

Several GenericValue functions take a const-reference to another GenericValue
only.  These functions can safely accept values with a different allocator
type.

The following functions are therefore extended by a SourceAllocator template
parameter in this commit:

 - operator==/!=/[]
 - HasMember, FindMember, RemoveMember
 - StringEqual

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

Some users may want to use RapidJSON with std::string objects.  This commits
adds an (opt-in) feature to include some basic support.  The implementation
uses std::basic_string<Ch> as generic string type.

Support currently covers:
 * construction
 * comparison

No special APIs for AddMember or PushBack have been added, as std::string
most probably requires copying (or an explicit StringRef() call).

By restructuring the call forwarding of the various operator== and
operator!= overloads, new overloads can be added by simply adding an
additional member operator==.

Additionally, the "Ch*" overloads are dropped in favour of an SFINAE
version that removes the pointer variants from matching the templated
operator== (see also operator=).

Add Doxygen documentation for configuration macros and error handling

Memory optimization