XD-906 Add aggregate counter monthly resolution

* Create an enum for the package and use this in preference to Joda DateTimeField. We don't need any of the latter's functionality anymore and it can't be used for months or years.
* Add query by month to aggregate counter.
  - Also changed the test to run across more date boundaries (e.g. 2013-2014) and fixed some bugs related to this.
* Add new aggregate options to shell
* Fix potential NPE in aggregate counter
  - When querying beyond with day or month resolution beyond the interval where data is actually recorded, a null value could be returned from the bucket store and cause a NPE. This is now checked and an array of zeroes is used instead.

Jira: https://jira.springsource.org/browse/XD-906