Added setup.py with 'test' command, along with some unit tests for xigt/core.py.
Initial support for extensions:
* core has a more consistent interface for collection classes (Igt, Tier, ...)
* RNC files are more consistent with separated X.content and X.attrs symbols
* Basic metadata just allows attributes and textual content
* Abkhaz example showcases extending tiers and metadata
* Abkhaz has XML codec extension, redefining just Metadata encoding/decoding