More sophisticated ABC notation support.

Support for key signatures, measures with persistent accidentals,
ties, and staccato. For example, the following uses two stacattos
and two ties, to play c# three times and c natural once:

play '^c.cc-|^c.=cc-|c'

Follows the ABC standard: dot before a note is stacatto;
dash after a letter is a tie; vertical bar is a measure bar.
Merge pull request #65 from PencilCode/moreabc

More sophisticated ABC notation support.