DB Transactions, Microphone

This commit adds a simple means to begin and end SQL transactions.

Furthermore, this commit adds the Arpk::Microphone object, which
represents a microphone-in line on the host machine.  In "push-to-talk"
mode, the microphone only broadcasts to its output line so long as it is
not muted; otherwise, samples must exceed a noise floor of 15 dB
(default, may be specified).

To broadcast captured sound, you must specify the output line via
#line_out=