* scm/lily.scm (postscript->png): new function

* scm/lily.scm (postscript->png): new function
(postscript->pdf): new function

* lily/paper-book.cc (post_processing): call
convert-to-{dvi,ps,png,pdf}

* scripts/ps2png.py (option_definitions): new file.

* lily/paper-book.cc (output): call output-preview-framework
(post_processing): new function. Do PDF/PNG conversion.

* scm/framework-ps.scm (convert-to-pdf): new function. Call
ps2pdf.
(output-preview-framework): new function. Generate a preview .ps