Tagstore API improvements
A few last API improvements to make this tagstore API a bit nicer, before we get too locked in.
Per @symbolist's suggestion, `Tag.tag` is now `Tag.name` which I think is much nicer
The API was missing a way to get an existing `Tag` without constructing it yourself, so I added a `get_tag()` method
Made the `Taxonomy` class a bit more pythonic by giving it some methods, so now you can do `taxonomy.add_tag('foo')` instead of `tagstore.add_tag_to_taxonomy('foo', taxonomy)`
The unit tests should cover everything quite well, so mostly review the code. You can also run the updated example code from `README.rst`.