(feature) Adds button matchers for default and cancel buttons.

[cguckes]

I coded this matcher for myself but began using it in multiple projects, so I figured I could contribute. Let me know if you want anything changed.

I got a few more matchers, but they're not really done yet, I might add them later, if you're interested.

[brcolow]

Hey sorry I missed this until now. Thanks for adding tests - that makes it very easy to review.

I restarted Travis as I believe the failures were unrelated flakiness. Looks good at a first glance though. Feel free to open more PRs with your other matchers, I would be happy to review them.

[cguckes]

No worries, I figured the built failed before, with the same error so just go for it :-) I will add the rest once I sanitized them and fixed the code style.

[brcolow]

Merged. Thanks a lot!