A lot of work toward enabling Stanford and hint visibility experiments:
- Specified levels on which experiments should take place.
- Switched to using ip addresses, rather than user ids, to determine
  experiment options.
A lot of work toward enabling Stanford and hint visibility experiments:
- Specified levels on which experiments should take place.
- Switched to using ip addresses, rather than user ids, to determine
  experiment options.
Added ip_hash field to activity_hints.
Improved tests.
Merge branch 'experiment' of github.com:code-dot-org/dashboard into experiment

Conflicts:
	app/controllers/application_controller.rb
Removed tests duplicated in prior merge.
Minor improvements.
Changed hashing to generate nil for bad/missing ip addresses.
Merge pull request #1092 from code-dot-org/experiment

Enabling Stanford and hint visibility experiments