Change User form to use Update for button if action is edit

I am also a new rails developer and I wanted to try a pull request. I noticed a small bug on your site. The user form for editing a profile still says "Join" even if you are already signed up. I Changed it to say "Update" if the user is editing the profile.

Please ignore if you are uninterested in this request and thanks for putting your code up on github.
:+1: