Fixed login form to reference USERNAME_FIELD

Was hardcoded to say 'username' when USERNAME_FIELD may be 'email' etc.
Updated LoginForm to dynamically set placeholder text

Placeholder text is now determined from the verbose name of AUTH_USER_MODEL's USERNAME_FIELD
Merge branch 'master' into custom-user-model-fixes
Merge branch 'master' into custom-user-model-fixes

Conflicts:
	wagtail/wagtailadmin/forms.py
Merge branch 'master' into custom-user-model-fixes
Added back 'your'
Merge pull request #705 from jsma/custom-user-model-fixes

Fixed login form to reference USERNAME_FIELD