Tune peak and IQR silence limits

Add some logging and reasonable presets/defaults for the new peak
and IQR delta silence limit settings.

We still don't have CLI for overriding these.
Comment out noisiest debug logs.
Update tests to use shorter chunk sizes

Update tests to use expected file lengths for the new, shorter
chunk sizes.

Remove hardcoded SILENCE_MIN and SILENCE_MAX since we're now using
deltas.
dBFS for silence limit specification

Change the silence limit specifications to decibels relative to
full scale (dBFS). This means, for example, that 0 dB is a full
scale wave signal, maxing out the sample width, and -6 dB is a
signal that only uses 50% of the sample width.

For reference, some of preset limits we're assuming are that
-15 dBFS peak to peak and -24 dBFS 50th percentile should pick up
conversational speech, while -27 dBFS peak/-36 dBFS 50th percentile
will pick up whispered speech.

Note that while we're stating dBFS for the level spec, we're actually
not measuring whether the signal comes that close to clipping.
Instead, we're measuring whether the peak-to-peak or quartile 1
to quartile 3 distance is greater than the relevant dBFS limit.

This makes us more robust against DC offsets in the input source
without requiring us to detect or fix any possible offsets.

Convert the default settings (whisper, quiet, and conversational)
to dBFS.

Provide a function to convert dBFS to sample value given a sample
width in bits.

Use this conversion function to convert the desired dBFS value to
sample values before passing them to the silence tagger generator.

Remove SILENCE_MIN and SILENCE_MAX CLI settings.

Add peak silence and IQR (i.e. 50th percentile) silence limit CLI
options, specified in dBFS.

Add CLI options to trigger the conversational, quiet, and whipser
silence level presets.
Clean up logging

Fix up logging to use Python 2.5 compatible formatters and
remove print statements.
Record statistics to file

Offer CLI option to record a "statistics" file containing dBFS values
for the peak and IQR ranges measured for silence detection. This file
can be analyzed to find the best silence limits to use for the
audio in question.
Add analysis script to show histograms of stats

Use pandas and matplotlib to raw histograms of the sample ranges
recorded in a stats file by SNARP. Useful for figuring out the best
silence dBFS limits to set.

N.B. The requirements file is probably over-large at the moment and
     should be tightened up.