let's users know they can use e-mail to sign in

Devise is currently setup to let people log-in with their e-mail address or username.  Which is great, but part of the reason i filed https://github.com/MolyJamCollective/MolySite/issues/44 was that it didn't actually say that I could use my e-mail to log in.

I conceded that issue due to Doug and Zane's points.  But I think my main annoyance with username login is having to remember exactly what username I was able to register with and it's always nice to be able to just use my email to get in.