GenericDocument: explicitly prohibit copying

See #201.

Minor (documentation) cleanups

Avoids implicit conversion from an `OutputStream` (or an allocator pointer)
to a `Writer` instance.

Use absolute path for the link to examples

Conflicts:
	include/rapidjson/internal/dtoa.h
	test/unittest/readertest.cpp

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the thirdparty performance tests are superseded.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

So the example link in documentation will always point to correct place.