Enable RTP keep-alive using empty RTP packets

Ignore-this: a73579a9783e2535c56de126646e23d1

darcs-hash:20140515164702-56b4d-93dbdced34f3f6dbf389db9c9f8cf8b7e7e76eb3
Store our Python object in the user_data field of pjmedia_transport

Ignore-this: 150c81d8e94c37094d971d4758f2f80b

darcs-hash:20140516093936-56b4d-bce1db45147a295352fdb36d2014a76292ac47e5
Simplified srtp_active property

Ignore-this: bcddd22ad8a63103189f407194e7607f

darcs-hash:20140516100221-56b4d-c2917fd7f9f540d2fce2087aa2932d8a9c9a5e56