Add to router-api test helpers.

* Add helper to stub out all registration.
* modify existing helpers to return the WebMock stub objects.
Router client no longer commits routes by default.

This reverses the sense of the option parameter to allow optionally
requesting to commit as well.  This is a breaking change, and will be
released as a major version bump.

The reason for this is that everywhere we use this at the moment, we're
registering more than one route, and therefore using the skip_commit
option.  It therefore makes sense to make this behaviour the default.
Merge pull request #162 from alphagov/update_router_client

Router client no longer commits routes by default (plus added test helpers).