Include dependencies for front-end unit tests

Using Karma, Angular Mocks, Chai and Sinon.
Add Karma test runner for frontend
Setup unit tests for the username directive

Also refactored the directive to remove the dependency on the
controller. The ng-model attribute is still required but only to set
the scope.
Remove ngModel dependency from username directive
Update user property on username directives
Exclude node_modules from pytest search
Run frontend tests within make test
Remove unnecessary watcher invocation
Merge pull request #1309 from hypothesis/angular-tests

Add basic test framework for the Angular app