Constants frenzy

This will also introduce a `RestfulBase` class to help (in time) alleviate the `RestfulEntityBase` class of all non-entity methods/functionality.
If you mistype 'GET' (instead of lower case) you will have unexpected bugs hard to track. With a constant you will get an error.

Also, all these 'get' are not independent from one another, so they should be centralized somewhere under a single_