added support for opus codec for audio in linux

Ignore-this: ba2e74fcb1f8ff2f494dd9ce45d63f36

darcs-hash:20140711042247-c21fe-397da18a049f245d67f7c32c4b43a4e373cdeb2c
added libopus in dependency list

Ignore-this: 2f937229db9035fb89bf71abd19a0772

darcs-hash:20140711042515-c21fe-31aee79fccdecd5d61afe057b31551eca21c07ad