Added `returnTo`

I've added and documented the option to add a `returnTo` parameter to the `/auth/provider/signin` URL which makes it possible to return the user to a particular link after a successful login.  See the updated README for details.  I've also updated other aspects of the README to tidy things up.  I've set the version to `0.12` for this to be releasable.  I can publish this to sonatype once it has been merged.