Add size attribute to Body and Liberation

Fixes #69
Remove Body.path, it's mostly useless now

Fixes #68
Register models to be searchable

TODO: subclass SearchEngine to disable save signals (they are useless to us) and add methods for manually adding and removing instances
Add context manager from watson to router

Defers search update until all models are saved
Understood docs a bit better, things make sense now

Ignore previous TODO