Incoporate WebRTC VAD code

[dhdaines]

Incoporate the more or less industry-standard VAD code from WebRTC. Still a draft as it is missing some really important features:

• Doesn't support 44.1kHz (which is pretty ironic since WebAudio only supports 44.1kHz...)
• Not integrated into the C API, and not very well integrated into Python
• Missing unit tests in C, because the original tests were in C++

Having this checked in will allow us to unit test it when we add these missing features.

[dhdaines]

Note that supporting 44.1kHz is fairly easy (if not really optimal) as the code to resample the input to an acceptable sampling rate (i.e. a multiple of 8kHz) is already present in the code, just not hooked into the actual VAD API.

The reason it can't be done without resampling is that the VAD code, for very good reasons, relies on filters designed and optimized for 8kHz sampled data. Arguably the right approach would be to have a parallel set of filters for 11025Hz data, which is, unfortunately, beyond my abilities in DSP.

[dhdaines]

Now accepting other sampling rates (70d848d) and has okay unit tests for VAD (93d02d5). Endpointer considered to be tested via Python but I'll add a small C unit test as well before merging.