Add Upload/TorrentGroup system, and tests for both.

See #15 for design ideas and rationale.
See batter/torrents_inheritance_tests/models.py for how this works
in terms of defining models, and
batter/torrents_inheritance_tests/tests/test_models.py for how this
works for accessing things.
Some travis tweaks, too
Try a package cache
Merge pull request #18 from wafflesfm/feature/torrentgrouping

Add torrent groups (and some Travis speedups)