Vocoder 2.0

The main innovations are twofold: decoupling the vocoder code into several steps (calcuate freq bands, bandpass the signal, generate envelopes, generate carriers, reconstruct), and adding a poisson click train as a built-in option for the carrier.  Have a look, @rkmaddox, and see if (1) it works as expected, and (2) it is organized in a sensible way.  I'm happy to do some refactoring to make this optimally useable.  @Eric89GXL feel free to chime in too.  

No new tests as yet, and since I changed the name from vocode_ci to just vocode, the old tests will fail at the moment.
Was there a reason we didn't just choose between using `filtfilt` or `lfilter` based on `zero_phase`? We're basically implementing a (slightly less good) `filtfilt` here. I don't remember...