Extra check in define-event-class patch

[Mike Solomon]

Hey all,
    I added an extra symbol check to my event class patch:

http://codereview.appspot.com/1867050

Also, I have a regression test that creates a spanner engraver from the
scheme end using this function (said regtest could also be turned into a bit
of documentation for the contributor's guide if people feel that'd be
helpful - I think it would given a recent thread on frogs concerning
engravers & their complexity), but I don't know if I should submit it as
part of this patch set or as a new issue on Reitveld.  Lemme know :-)

~Mike

0002-Symbol-checking.patch
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0001-Create-ly-define-event-class.patch
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