Disable submit button only if at least one field is empty.

* Add angular-js $watch
* Comment out other listeners.
Display button behavior when link disabled.
Increase font size for registration messages.
Merge branch 'master' into improve-sign-up-validation-errors-26
Render devise errors rather than custom errors for username/email.

* Show errors only when user tries to save.
Increase line-height of description.
Clean up userForm.js
Display email/password validations only on Submit.
Merge pull request #27 from vitaliyg/improve-sign-up-validation-errors-26

Improve sign up validation errors 26