Adding the users endpoint to return all users and added the test
Adding the /v1/users end point to return a list of all the users
Changing the PUT endpoint to take an id
Changing the PUT endpoint for skills to take an id
Flipping the arguments in retrieveAll
Adding assertions to test the results of retrieveAll
Fixing some miscellaneous stupidity
Adding a method to user serializer, adding a test and changing the endpoint to use said serializer
Fix issues in API endpoints causing FATAL errors

Several missing `$id` parameters in controller functions caused
fatal undefined errors to be returned in the API response from
Slim.

The way user accounts were being serialized in a collection was
causing FATAL errors because each `User` was being converted to
an array and then an attempt was made to access the
`learningSkills` and `teachingSkills` properties of the object.

Fix uses of `users` table which is actually named `user`.
Fix to return all partnerships
Merge pull request #74 from phpmentoring/user_list_endpoint

User list endpoint