Add Application ratings to API

[tomkonidas]

We add the following routes:	METHOD	path
GET	/api/v1/applications/:application_id/ratings
GET	/api/v1/applications/:application_id/ratings/:id
POST	/api/v1/applications/:application_id/ratings

For now I have set all reviews to be approved. We can adjust this later on to needs_review as a default if we feel we get too much spam going on.

This is what the body of an application looks like:

Create request Types

Field	Type	Required?
application_version	String	true
application_build_numebr	Integer	true
rating	Integer	true
notes	String	false
google_lib	Enum (none or micro_g)	true

Updated

All application objects now return an average rating (2 decimal point precision) or null if there are none

By default i filter out only approved reviews.