Add option to indicate frets by letters in tablature

[Trevor Daniels]

I've just posted patch set 3 to Reitveld, after quite
some struggles with juggling regression-testing and
doc-building in a too-small virtual ubuntu system.

See http://codereview.appspot.com/164063

The main change from patch set 2 is to generalise the
input format of the context property fretLabels to
permit markup, and changing the property's name from
the previous fretLetters.  Thanks to Carl and Neil for
help with this.

I've added two news items, and dealt with the other
minor comments.

Reviews requested.

Trevor