Static files injection to SecureSocial

Added some new val in RoutesHelper to provide the paths to some css/javascript files (like JQuery or Bootstrap)
used by SecureSocial. By default the use the files provided by SecureSocial.

This lets users to replace these files without having to create a new Template object

A new custom CSS file has been added (default: empty path) to allow injection of custom styles to SecureSocial pages

Notes: this could be probably extended to all other custom images/icons of SecureSocial to facilitate integration with user apps

Please check if you like or propose changes to fit to your requirements :)

Updated documentation too.

(note: I'm not in the best shape tonight and I never used erb for documentation before today, so please check I did not mess anything hehe)