Calculate statistics and update the authorities list page

* Adds a new table for AuthorityStatsSummary
* Adds a rake task to calculate stats per authority as a whole and on a per category basis (based on data already loaded into the database)
* Uses some ruby magic (and a horrendous if statement) to make `percentage_#{attribute}` methods for the stats counts
* Fixes a bug where a single controllers to override in the theme was hardcoded, now loads everything ending in _controller.rb in the theme's controllers directory
* Overrides the authority list controller
* Example wire-up of authority list page - more than happy to take this bit out if it makes implementing #80 harder

Closes #30
Closes #6 
Good idea to introduce constants - should ease the pain if we decide to switch from "In progress" to "pending" (#97)