add chained expectation descriptions to the default descriptions of custom matchers with fluent interfaces

this fixes #532
Hmm, this makes me realize that this change is going to make sub-optimal descriptions (and potentially, failure messages) for existing custom matchers that use `chain` and were overriding the description to compensate for hte fact that the chained part wasn't being included.

Is there a way we can avoid this problem?  (I can't think of a solution that I like).