Slick demo

Hi!

I've started on a slick demo, since the in memory demo doesn't tackle a lot of the problems with how to extend the Identity and implement all the needed methods.

I'm not entirely done yet and I need some help.

On this row
https://github.com/leon/securesocial/blob/slick-demo/samples/scala/slick/app/models/User.scala#L34

I'm not shure how the Identity get's populated. I'm guessing that depending on how you choose to login, we maybe need to convert the hasher, password, and salt to Option[String] since these don't get populated if we are using one of the oAuth providers.

The same goes for
https://github.com/leon/securesocial/blob/slick-demo/samples/scala/slick/app/models/User.scala#L26-L28
I'm unsure on what these should be.

One other thing that I thought wasn't to good was that by extending Identity, identity already has a field called id, this meant that I couldn't use the normal naming schema for the database and had to go with pid (primary id).

Great job on the plugin!
I could also throw together a demo for mongo either using reactive mongo or salat