rapidjson

Closes #1643 (#1644)

This change comes up with compile time pre-processor directives to
tune the behavior of rapidjson wrt memory consumption. The idea is to
allow each module using this library to choose the right defaults based
on how it consumes memory and what performance it expects.

1. RAPIDJSON_DEFAULT_ALLOCATOR: If defined allows you to choose
	CrtAllocator over MemoryPoolAllocator. If it is not defined, chooses MemoryPoolAllocator by default.
2. RAPIDJSON_DEFAULT_STACK_ALLOCATOR: If defined allows you to choose
	MemoryPoolAllocator over CrtAllocator. If it is not defined, chooses CrtAllocator by default.
3. RAPIDJSON_VALUE_DEFAULT_OBJECT_CAPACITY and RAPIDJSON_VALUE_DEFAULT_ARRAY_CAPACITY: If defined and set to a
	value, uses that value for default number of objects/array elements to be pre-allocated. If not defined,
	uses value of 16: the current default.

Verified that all tests pass.