Issue 650

This is a fix for issue #650.  Event handlers can now be set multiple times, without clobbering the previous definitions.

I couldn't find an easy way within meteor itself to test these changes, but I've tested them with meteor-mocha-web.  Clone https://github.com/mad-eye/meteor-eventhandlers.git and run `runme` (you might have to `mrt update` first).

Once this gets reviewed, I'll update the Spark documentation.