[WIP] Add more Yardoc

This is a WIP.  Added documentation for the aggregate API, along with adding some missing specs. 

Please pay attention to the filter documentation.  Filters in general could use some improvements.

YARDOC: http://rubydoc.info/github/envato/double_entry/aefe8e/frames

More things I'd like to do:
- [ ] Create a time_range API to create a TimeRange object & return it (probably in separate PR)
- [ ] Document aggregate_array after getting comments back on aggregate
Sadly, it doesn't appear to allow filtering without scopes, so this is the best we can do for now!