Allow customization of RapidJSON namespace

Document traits of types using <type_traits> and static_assert in tests

The tests state the current traits of types Document, Value and
StringBuffer. There are slight differences between them. It seems like a
good idea to extend this approach across more types, and to review the
expected traits across the board.

MemoryPoolAllocator, Stack: lazily allocate Allocators 

DocumentMove tests: compiler error due to -Wsign-compare

Add unit tests for StringBuffer

On GCC 4.9, the documenttest.cpp fails to compile in C++11 mode,
as the MoveConstructor/Assignment tests contain left-over
comparisons between signed and unsigned expressions.

In order to make the constructors more efficient, especially
in the context of C++11 move semantics, the (dynamic) allocations
in MemoryPoolAllocator and Stack should be performed lazily.

Move the allocations to the first use of the allocator in both
classes.

For consistency, all member variables should follow the same pattern.

Add customization macros for global new/delete

As mentioned in #181, some environments may require adaptations to
the internal calls to the global `new`/`delete` operators, like
adding explicit `NULL` checks to `delete.

This patch adds two new macros
 * RAPIDJSON_NEW(x)
 * RAPIDJSON_DELETE(x)
to allow user-defined expressions in these cases.

This fixes #181 in an alternative manner.

Drop thirdparty libraries and minor cleanups

With the implementation of the more elaborate benchmarking
suite at https://github.com/miloyip/nativejson-benchmark,
the thirdparty performance tests are superseded.

Clarify documentation on RemoveMember, EraseMember ordering

Use 'deprecated' instead of 'depreciated'

Issue123movesupport

Added MemoryPoolAllocator to GenericDocument moveunit tests. Added comment in GenericDocument move assignment operator explaining why the static_cast is needed to move the base class.

GenericValue: improve documentation for operator[]

GenericValue: improve operator[] disambiguation

In the original disambiguation fix for `GenericValue::operator[]` (#170),
the documentation has been missing, which led to quite badly rendered
Doxygen pages.

During a cleanup, I've realized that a much simpler disambiguation is
possible:

````cpp
  GenericValue& operator[](SizeType idx); // array
  template <typename T>
  GenericValue& operator[](T* name);      // object
````

This approach works, as non-template functions are preferred over
template functions.

In order to improve the error messages, the pointer type is restricted
to `(const) Ch`.

Update `tutorial.md` to drop the ambiguity warning.

Demonstrate CRTP in SAX example

Disambiguate GenericValue's [0] and ["string"]

Closes: https://github.com/miloyip/rapidjson/issues/142