Extract versioning from Crepe core

API versioning will be moved to [crepe-versioning](https://github.com/crepe/crepe-versioning).
This closes #56

Signed-off-by: David Celis <me@davidcel.is>
I actually really liked the ability to override `request_class`, and it seems like this is another place `crepe-versioning` has to override an entire method. Any reason we shouldn't leave this in place to allow modules the ability to customize the request object?