Added SDPNegotiator class

Ignore-this: 493b766196a3e90d882400ce3cbd9bf7

darcs-hash:20130221100910-56b4d-338b974c1d09873efbfe01814f32dfc2937b5c79
Avoid computing time.time() for each timer that needs processing

Ignore-this: 9774f0e23611af65b59cbf0fb44dff93

darcs-hash:20130222155235-56b4d-1e8d827fac6daf8ab34d417aad030d1d59953361