Add Post entity and spec, refs T225

Test Plan:
```
bin/phpspec run spec/Ushahidi/Entity/PostSpec.php
```

Reviewers: dkobia, rjmackay

Reviewed By: rjmackay

Subscribers: lkamau, eyedol, PapaLee

Maniphest Tasks: T225

Differential Revision: https://phabricator.ushahidi.com/D97