Recursive groups & text files.

The previous release only used a boltdb file to store all the time tracking information. This release stores historical information in text files located in a data directory. Boltdb is still used as a cache. The commands are very different from the previous release. The readme is up to date with all the changes, so check that out for documentation. I've been using this release for about 2 weeks before tagging it.

The API will be pretty stable starting with this release. I expect future releases to include features and stability improvements instead of rewrites.