Merge from vendor branch TNF:

Merge from vendor branch TNF:
Import celt-0.5.0.

CELT is an ultra-low-delay audio codec designed for high-quality
communications.  Its potential uses include video-conferencing and
network music performance.  The code is still in early stage, so it may
be broken from time to time.

NOTE: The API is not frozen yet, so it is different from one version to
another.