Added versions API and tests

[louispt1]

Very simple API for versions, with a basic url builder. I expect this will need updating.

[noracato]

Let's finish this one when we start to implement the version switching in model and engine. I think we need either more (model engine collections specific) or less (only tags). Probably the urls should come from the Settings, which means a lot of the logic can be incorporated into the model instead of the controller. Let's see!