Improve speech handling

[gcampax]

Even when the wake-word works, Almond's speech handling code is not very user-friendly. Almond shuts off the microphone while its speaking, and it cannot be interrupted. Any follow up command needs a wake-word again, even a simple "yes" or "no". Parts of the wake-word sound are often captured by the Speech To Text code, leading to broken chunks at the beginning of the command sent to the NLU.

We should clean up all of that.