Allow versioned client models to be used concurrently

[dmcinnes]

For example:

API::V1::Post.find(1)
API::V2::Comment.find(2)

Since this is an edge case for development or migration purposes.
Associated classes will adopt the version of the parent (unless we offer an option to fall back).

Will need to set_prefix on a per class bases through inherited.

[dmcinnes]

Moved from https://github.com/att-cloud/daylight/issues/2