Abstract getCollection into Ushahidi_Repository

Summary:
- Abstract getCollection into Ushahidi_Repository
- Use abstracted getCollection method in Media/Tag/User repos

Test Plan:
./bin/behat -c application/tests/behat.yml application/features/api.users.feature
./bin/behat -c application/tests/behat.yml application/features/api.tags.feature
./bin/behat -c application/tests/behat.yml application/features/api.media.feature

Reviewers: dkobia, shadowhand

Reviewed By: shadowhand

Differential Revision: https://phabricator.ushahidi.com/D221