Updates for VS2013, added 64-bit types, minor bug fixes

Fixed warnings for VS2013
Corrected const correctness
Added conversion to/from strings 
fix 64bit vs2013 warnings
size() now returns int to be consistent with the rest of the api
Added unsigned int, long, and long long
Clarified conversions

These are the changes that were necessary to use it in my application. Overall a very useful library!