[PATCH] Fix issues with episema formatting (Issue #189/comments in docs)

[Neil Puttock]

Hi everybody,

This patch allows an episema to be typset over a single neume (issue
#189) and ensures it can be positioned correctly inside the stave.
Since the requirements of an episema are slightly different from
standard TextSpanner grobs, I've decided to create a new engraver for
this purpose: this ensures simultaneous events (i.e., start- and
stop-spans on a single neume) aren't junked, and allows the correct
positioning by adding the acknowledged NoteHead grobs as
'side-support-elements and setting 'quantize-position.

Please review:

http://codereview.appspot.com/186189/show

Thanks,
Neil