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
do you think it is a good idea to be specific about the method that this spec is testing, i.e. adding one line `describe '.aggregate'`?