Forced email to be lowercase

I have seen that most of my user insert an upper case email or some
letters are capital.
In this way all the email in the db are correct.
Merge git://github.com/pyrocms/pyrocms into 2.2/develop
Merge git://github.com/pyrocms/pyrocms into 2.2/develop
Merge pull request #2552 from ChristianGiupponi/2.2/develop

Forced email to be lowercase on registration