ZRTP updates

[Werner Dittmann]

Thanks to Mikael's fix to the libminisip configuration
files the ZRTP can be compiled again.

I've just checked-in some updates and fixes to ZRTP.
The current status:

- inter-operable with twinkle (based of GNU ccRTP)
- inter-operable with Zfone-beta2 - at least most of the
  time. Zfone uses a strange method to use the RTP packet
  sequence numbers. This may cause failure of the key
  negotiation. The next version of Zfone fixes this problem,
  thus I didn't spend too much time to implement a workaround
  in minisip.
- an of course inter-operable with minisip :-)

Regards,
Werner