Some Comm fixes.

This fixes a few issues with `comm.id` no longer existing, and also has a temporary fix for the strange `metadata` method dispatch issue.
@JeffBezanson, @stevengj This is an issue that @shashi ran into. I wonder if you have any ideas about what's going on here. It seems to be dispatching to the wrong method, despite `@which` returning the correct method. I added this work around, but it's just a temporary fix so I can show react at JuliaCon.

Is this a julia bug, or am I missing something subtle?