Implement a mock version of the api

[amihaiemil]

For unit testing, we should also offer a mock version which will work with a JsonBuilder behind the scenes, instead of making real HTTP calls.

When unit testing, instead of using a real instance:

VersionEye api = new RtVersionEye();

Users should use

VersionEye mockApi = new MkVersionEye();

[amihaiemil]

done