AudioDesc: suppress copy constructor

* This definition raises an assert when push back a Dummy stream into a
vector (see Transcoder class): assert( m_codecContext != NULL ).
* Keep symmetry with the VideoDesc: no copy constructor specified.
StreamTranscoder: fix desctructor when dummy stream

* Need to know if it is a dummy stream or something else when destruct
StreamTranscoder:
     * _inputEssence references to a Dummy created in Transcoder if
dummy: not delete it in StreamTranscoder!
     * new _inputEssence in StreamTranscoder if not dummy: delete it in
StreamTranscoder!
StreamTranscoder: complete destructor

* Delete _sourceBuffer if necessary.
Merge pull request #21 from cchampet/fix_dummy

Fix dummy stream