Preliminary overhaul of cuesheet interface.

The old interface didn't handle sheets containing tracks with multiple
indexes well at all and had always been a bit of a hack.

The new version has get_cuesheet/set_cuesheet/read_sheet only
deal with Sheet objects which should make the process less painful.
Restore set_cuesheet() to FLAC.

Update tracksplit to take advantage of new cuesheet interface.
Add write_cuesheet/write_tocfile functions to cue/toc modules.

These take sheet objects, filenames and a file handle
and output a (hopefully) properly-formatted
.cue or .toc file.
Update trackinfo to work with new cuesheet interface.

Add a number of fixes to improve cuesheet parsing,
including support for WavPack and True Audio formats.
Update documentation to cover new interface.
Fix unit tests to handle new cuesheet interface.