Feeds support

Pretty sure this doesn't do anything. Yes, it does make the Serial field visible in the list of Target fields; but when user is created it simply uses the next serial number as opposed to the ones being imported.

Is it possible this can not be done with Feeds import and only way to accomplish this is to be able to disable serial function until after migrations?

After discussion with Feeds maintainer (@MegaChriz), he agreed this could not be done with a basic Feeds target as the Serial module will override the import value. SO i added a tweak to the getSerial() call in this module to not insert its serial value if there is already a value set at that point (like when coming from import).

Attached is updated patch since previous patch is failing. Also removed MessengerInterface and protected variable since we are not using it anywhere.

The code is working fine and import works.