Add UshahidiApi/Endpoint, standard parse -> usecase -> format sequence, refs T666

Summary:
- enable UshahidiApi for phpspec and autoloading
- create UshahidiApi/Endpoint class
- create Ushahidi/Usecase interface
- define `endpoint.tags.post.collection` for `POST /tags`

Test Plan:
First: `bin/update`
```
bin/phpspec run
bin/behat -f progress -c application/tests/behat.yml application/tests/features/api.tags.feature
```

Reviewers: rjmackay

Reviewed By: rjmackay

Subscribers: dkobia, eyedol, bodacea

Maniphest Tasks: T666

Differential Revision: https://phabricator.ushahidi.com/D314