New PulseAudio driver for speech-dispatcher

[Bill Cox]

I've build a new pulseaudio driver for speech-dispatcher.  Performance
reported from users so far is good, similar to the new libao driver,
but with better pulseaudio buffering parameters.  On some system, the
default libao pulseaudio buffering parameters causes problems with key
echo.  I've made those parameters visible to the user through the
speechd.conf file, so they can play with them to see what works the
best.  The only one I find critical to set to non-default is tlength.

The new pulseaudio driver is based on the libao.c driver from Luke,
and the ao source code.  Basically, the ao library simply calls the
pulse-simple API, and I just cut and pasted those calls into libao.c.
Thus, the performance is similar.