Bugfix: return the correct gateway.

After this fix we always returned the first gateway, no matter what the
user asked for.
Allow to create the class using a specific offset.

This is useful for testing purposes, so we can be consistent with the
distance calculation.
Remove test exclusion for nose.
Bugfix: timezone calculation.

Also use int notation instead of datetime.timedelta.
Add test for vpngatewayselector class.
Merge remote-tracking branch 'ivan/feature/2962_test-vpngatewayselector_rev1' into develop