add Again and ContextTerminated exception subclasses

and _check_rc utility

- [x] makes catching the most common exception cases easier
- [x] breaks nothing
- [x] fixes exception when zmq_poll is interrupted, e.g. terminating IOLoop.

[forcing travis rebuild]
get errno before making any Python calls in _check_rc

why? Because I said so, that's why.

(I have no idea why, but Python 3.2 seems to need it)
test new exceptions

poke @travisbot
Merge pull request #308 from minrk/exceptions

add Again and ContextTerminated exception subclasses