Use more stdint.h types and document disabling stdint.h

Summary

* Include #145
* Rename `RAPIDJSON_NO_INT64DEFINE` to `RAPIDJSON_NO_STDINT` and revise documentation to address @pah's concern about unconditionally using `int32_t`/`uint32_t`.

As I discussed in #145, I think this change improves several things:

1. Better type documentation
2. The name of the macro `RAPIDJSON_NO_STDINT` more accurately describes its purpose than `RAPIDJSON_NO_INT64DEFINE` did. It disables the `#include <stdint.h>` and doesn't only disable the definition of `int64_t`/`uint64_t`. Of course, this is more significant with the usage of `int32_t`/`uint32_t`.

@pah had the concern that the type definition `int32_t` implied the implementation type would be restricted to 32 bits on a 64-bit system. I'm certain all 64-bit systems will use the most efficient implementation type for `int32_t`. To check, I looked at `stdint.h` on Mac OS X and Linux as well as `msinttypes/stdint.h`: all of them use `int`.