Refactored media streams to be plug and play

Ignore-this: 1832d9a3ea4fd8e696a087f6f82bfb1
- automatically register media streams to a stream registry
- register media streams with a given priority which allows user defined
  stream implementations to overwrite the default stream implementations
- added a new class method new_from_sdp to the IMediaStream interface,
  that builds a stream for an incoming request or raises an error if it
  can't handle the stream or the stream is invalid
- removed validate_incoming from the IMediaStream interface as it was
  superseeded by the new_from_sdp class method
- the session no longer needs to know how to interpret the incoming SDP
  in order to find out which media stream classes to instantiate, it only
  has to try the media streams from the stream registry in order until
  it finds one that can handle the stream
- reorganized all stream modules into the sipsimple/streams package

darcs-hash:20091031064045-b22ec-796319842c01f73be76bab29942b742a80d18db9
when allowing a watcher remove it from blocked and politely blocked lists if applicable. A watcher will only be in one single list. Fixes issue #60

Ignore-this: d16c715c0d7afa2c42fac1fc997ccad5

darcs-hash:20091103121340-4dec5-808bd3594763c1f984e919ac5e5d9c25c4e4a8ee