Post set api

Initial Implementation to fix issue #57 

- List posts within a set
- Add existing post to a set
- Delete post from a set
- Behat test for listing all post attached to a set
- Behat test for finding a particular post attached to a set
- Behat test for removing a post attached to a set 
- Behat test for adding a post to a set

Next writing necessary doc for the resource.

ping @rjmackay or @kamaulynder for a code review