Libgatekeeper should provide an API for instantiating MifareTags

The current API of `libgatekeeper` has functions which take a `MifareTag` argument, but no way to create one. Could you describe a little bit about what the MifareTag represents and how a client of libgatekeeper should expect to obtain/use one? I feel like a goal of this library should be a clean encapsulation of all of the underlying libfreefare/libnfc types and functions, so that gatekeeper logic built on top of this should be able to accomplish everything using only libgatekeeper functions.