Added code to pull down and load GA4GH schemas in Python.

This adds a script which pulls down avro-tools and the GA4GH schemas, and prepares them for use in Python. I've also included a unit test which demonstrates it's use.

One bit to note: unlike the Java AVRO bindings, empty arrays/maps and optional fields must be explicitly nulled out. Else, the avro writers will throw a schema mismatch exception.

Also, I added a .gitignore; I've set it up to ignore emacs temp files, and to prevent *.pyc files from being checked in.