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/4aa470/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
I stared at it for a while, and we don't appear to use it anywhere.  It's saved as an instance variable and then never used.  It's an argument into LineAggregate...but the calculate method in aggregate always passes `nil`.  I can keep it here if we want though, but it felt a little misleading!