UK: util function to return number of submissions for a given xform
UK: view to display submissions per day
UK: add  mongo aggregate functionality to parsed instance
UK: use mongodb aggregation for query
UK: submission counts grouped by a given field
UK: set form by id_string
UK: api endpoint /stats/submissions
UK: use api function for submission stats
UK: add link to submissions stats
PLD: refactor group dict building
PLD: use if-else over two ifs
Merge pull request #18 from kobotoolbox/submission-stats

Basic statistics: binned frequencies