Renamed "url" to "key"

It seems that using the term "url" for cache keys implies that they are primarily URLs, and secondarily cache keys — when in fact the opposite is true in practice.

I explained this in the following comment on issue 4:
https://github.com/TheInvisibles/ActiveREST/issues/4#issuecomment-45418786

This pull request renames "url" to "key" throughout the system, which I think will make it more obvious how to use ActiveREST for new users.