Use templates for email, html and txt.

Using templates for the email body makes it easier for users to
customize the login email.
Accidental comma
Remove send_login_email from save method.

The change from commit 07faaeec causes two emails to be sent if we send
the email from the save method. This commit removes that from save and
makes it a separate explicit step. The only alternative would be to find
a way to pass our next url into the authenticate function but that seems
like it's getting messy.
Merge pull request #10 from smajda/master

Use templates for email, html and txt.