transifex.net is deprecated, use transifex.com instead

http://support.transifex.com/customer/portal/questions/1160404-improve-ssl-security-for-transifex-net
tx client is giving a message about it too

Also add translation type 'PO'.
Merge pull request #201 from zas/transifex_update

transifex.net is deprecated, use transifex.com instead