General upgrade

Hi Mihai,

thank you for the plugin.

I'm using it in a little project and I had to modify it a bit.

This is the first of 3 PR, others are on specific feature branches.

My changes in brief:

- general upgrade for Grails (2.2.0), deps and plugins

- added exception for a better error handling

- added a very basic (hope to improve it) taglib which shows if a user is logged in with a given provider

- introduced Spock for testing

- skip link/create account if user is already logged in

Hope you find something useful to apply.

Regards,
Enrico