Tweak ID3v2 tag skipping routines.

Try to ensure they check a few more things for correctness.

Also update unit tests to ensure sync-safe things are actually
sync-safe.
Simplify FLAC's offset handling slightly.

The decoder's offsets() method no longer relies on ftell
to keep track of the current position.
Shift ProgressQueue's output text from stderr to stdout.

This might be a little more intuitive.
Update BitstreamReader to take strings as input also.

It now takes files, file-like objects with .read() and .close() methods,
and plain binary strings as possible input objects.
Minor formatting updates.
Update tracktag unit test.