events: Don't assume .domain is a Domain instance

EventEmitter assumes that anything with a .domain property is an instance of a
Domain.Domain.  This clobbers any EventEmitter instances/inheriters that make
use of the fairly generalized property name.  Change is backwards compatible
with existing interface.  New test included.

Fixes #3922

I know pull request 3932 is already open, but this would allow this to be fixed
in the next v0.8 release as opposed to 1.0. 

https://github.com/joyent/node/pull/3932