Fix #37: Change validation msg if email present.

In case the user's email adress is already present with another account
during sign up, notify the user that a manual connection of this social
account is needed in order to use it for signing in to the existing local
account.
Add Transifex configuration.

Add a configuration file for Transifex (a collaborative translation site) and
add a *.po file for the source language (en).
Changed translation.