Support Value to capture the size of the integer

This is the fix of the issue #38.

* Renamed IntegerValue to IntegerClassValue. This class abstracts msgpack's integer-related Value(uint8, uint16, uint32, uint64, int8, int16, int32, int64).
* Introduced BYTE, SHORT, INT, LONG into ValueType as subclasses of IntegerClassValue.
* Support auto conversion of IntegerClassValue when readValue() is called.