Merge Rests Engraver

[Jay Anderson]

https://codereview.appspot.com/321930043/

It was requested sometime last year that I attempt to get this into
lilypond proper this is my attempt. It creates a scheme module for putting
all of the merge rest functionality. I didn't see any precedent for this,
so let me know concerns. If this approach doesn't seem right (e.g. feel it
should be part of the rest engraver) I can either add a TODO to that effect
or I can drop this patch.

When running git-cl it failed to create a new ticket number to track the
issue (the instructions say I need to request access:
http://lilypond.org/doc/v2.19/Documentation/contributor/git_002dcl#configuring-git_002dcl).
There is an existing issue (
https://sourceforge.net/p/testlilyissues/issues/1228/). Let me know what
more I need to do there. Thanks.

-----Jay