Prevent duplicate signup overwriting data

Prevent a duplicate signup request from overwriting a logged-in user's
e-mail address and password.
Convert all email addresses to lower-case

Prevents duplicate accounts with identical e-mail addresses, and users
unable to log in if typing their e-mail address with the wrong case.

WARNING: This change is incompatible with existing databases containing
non-lowercase user e-mail addresses!
Merge pull request #6 from dfearon/master

Prevent duplicate signup overwriting data