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.
The `metadata(::Signal)` method is here: https://github.com/shashi/IJuliaWidgets.jl/blob/master/src/IJuliaWidgets.jl#L44

In the a previous build of IJulia, printing something (e.g. an empty line) inside `metadata(::Any)` used to, weirdly enough, fix this problem.