[CELT-dev] Many changes recently

[Jean-Marc Valin]

Hi Everyone,

I just merged a lot of changes in the CELT master branch. This should
solve the following issues:
- tonal artefacts (birdies) at low bit-rate
- tonal artefacts in stereo at all rates
- low-frequency noise (roughness) on low bit-rate speech

The code has been simplified a lot, but there's lots of changes, so
please give this a try and see if anything got broken.

Cheers,

	Jean-Marc