Use a relational database

[tyarkoni]

Grabbit started out as a minimalistic tool for managing/manipulating filenames, but we're now at the point where the internal model is getting too complicated to justify a pure Python solution. A lot of the queries are now complex enough that a SQL-based solution would be much more elegant and performant. I'll likely reimplement grabbit to use peewee internally in the next few days. The API should change minimally if at all--though I'll probably use the opportunity to introduce some other breaking changes (e.g., #57 ). Comments are welcome.