Three new APIs are added for JSON object type.

With quite some discussions on naming of the APIs, finally adds 3 new APIs:
* `bool ObjectEmpty()`
* `SizeType MemberCount()`
* `RemoveAllMembers()`

APIs similar to `vector::capacity()` and `vector::reserve()` are not added for member, because [some possible implementations of associative array](https://github.com/miloyip/rapidjson/issues/102) may be considered in the future and these APIs may not be suitable for those situations.

Fixes #116