Pending groups in formatters

This fixes #131 for me.

Fixes integration with formatters written for RSpec. RSpec emits start,
then pending no matter how the example is marked as pending and some
formatters out there[1] requires this reporter to follow the same
convention.

I decided that I wanted to implement the special case with a message rather than another branch inside the already too big `process_mocha_event`. I think the visitor pattern fits nicely here. :-)

I was not sure how to integration test my specific problem so I did not do it. I did, however, document with a comment why the special behavior is there and it is possible to just frame this change as "conform to specification" rather than "fix problem with external formatter".

[1]: ci_reporter is such an example