Make it easier to use a custom backend

[SpaceK33z]

It should be a bit easier to use a custom backend. This package is clearly suited towards Binder, our Python backend, but we can at least provide hooks to customize the backend handling.

[SpaceK33z]

This is related to #2.

[stam]

It would help a lot if we can override the request instance that models and stores use.

[SpaceK33z]

I worked on this Sunday. You can use the separate-api branch if you want this. It is essentially finished.