Add Yardoc to Reporting API

 Added documentation for the aggregate/aggregate_array 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/0b1d55/frames

More things I'd like to do:
- [ ] ~~Create a time_range API to create a TimeRange object & return it (probably in separate PR)~~ 
- [x] Document aggregate_array after getting comments back on aggregate
- [x] Wait for https://github.com/envato/double_entry/pull/24 to merge, merge & move commentary to new module
Yeah, doing any major refactoring wasn't the goal of this PR, so feel free to throw something on the Trello board for this!  This change just came about due to the fact it's an option for the aggregate method and wasn't being used (which I discovered when trying to describe it!)