participant() is responsible for caching lookups
Refactor goal counting code

Only the actual recording of a specific goal against a specific
alternative now varies based on the user type
Django 1.4 datetime support
Make _get_all_enrollments return a nametuple
Fix bug in deletion logic
Record number of days visited
Very dirty graphs for freq distributions
Hide excess points
Visualise as cumulative value
Make graphs toggleable
Clean up freq fixups
Fix divide by zero
Fix max of possible empty sequence
Merge pull request #96 from mixcloud/ts_histogram_graph

Retention tracking and frequency graphs