Fixed raising incorrect exception type

Ignore-this: a27a8b3ef6faf9dc8fb44eac647dadc9

darcs-hash:20130715122646-56b4d-d3212338dcf7a8bccc2a2df6492fbae0ee64d684
Made ec_tail_length mandatory when creating an AudioMixer

Ignore-this: fb24d7263c1524de00dcc76d9747689b

darcs-hash:20130715132346-b22ec-b9a90e63a68b42b5d3c99bb062e7ef438ba30738
Added enabled setting for echo canceller and echo_canceller settings group

Ignore-this: 24cb410b8fe6a41dc4b8a3cf2ba500e4

darcs-hash:20130715133844-b22ec-3ae845f7a061f5a43852e94c4c64ed12ade43b3e