Add get interface for token expiration

With this, library users can now query when the token will expire.

Additionally, fixes the constness of one of the function methods.

Implement remaining API

The initial release left out a significant API implementation, get_claim_string, which returns the claim's value as a string. Without this, introspecting tokens is quite difficult!

This also includes the code necessary for building the library on Mac OS X.

Light-of-day SciTokens C++ Release!

v0.1.0 celebrates the very first version of the SciTokens C++ library worth seeing the light of day.

It features most of the same concepts as the Python library - the SciToken, the Validator, and the Enforcer. This should be worth a look from anyone previously embedding the Python library into a C or C++ application (the library actually exports a C-linkage API, even though the implementation is in C++).

As it's not a "1.0" release, don't be awfully surprised to find things like memory leaks, bugs, or crashes. Bug reports are always appreciated -- and pull requests even more!