Tracker 836: Add facility to change output file-name for a \book block

[ian]

Reviewers: Neil Puttock, carl.d.sorensen_gmail.com,

Message:
The latest version of this patch is now ready for review.

Cheers,
Ian

Description:
Tracker 836: Add facility to change output file-name for a \book block
or to set a suffix to prevent multiple files over-writing each other
during a
compilation. This change allows user to do this via functions rather
than having to do so by using set! and define on parser variables in
Scheme.

Please review this at http://codereview.appspot.com/150044

Affected files:
  M lily/parser.yy
  M ly/init.ly
  M ly/music-functions-init.ly
  M scm/lily-library.scm