Add multiplier capability to bitstream's parse/build routines.

That is, if one wants 100 signed bytes, you can specify "100*8s".
This should cut down on cases of having to do .parse("8s" * 100)
at the Python level in order to fetch several values at once.

The .build() routines also take an iterator of values to be
output rather than something that needs an index.
Add more comprehensive unit tests for bitstream's parse/build routines.
Add documentation on parse/build multiplier field.

Update some Python routines to take advantage of multiplier.
Add basic stream conversion options to track2track.

This allows one to adjust sample rate, channel count and
bits-per-sample during conversion.

It might not be a bad idea to add this feature to other
conversion programs (dvda2track, tracksplit, etc.)
but I'm not sure how much those would actually use it.