drop-in subset of tornado 3.0 into zmq.eventloop.minitornado
use IOLoop from mini tornado or tornado proper

removes base implementation from eventloop.ioloop

eventloop.ioloop is only an API package now.

Installing the zmq IOLoop in tornado 3 only uses public APIs now, yay!
bump version to 13.1.dev

because tornado change constitutes 'new features'.
cleanup a few imports from tornado 3 upgrade
fix stack_context import in zmqstream
Merge pull request #356 from minrk/tornado3

upgrade to tornado 3

ships minimal subset of tornado in zmq.eventloop.minitornado

If tornado 3 is present, the zmq IOLoop is a minimal subclass of tornado's own IOLoop,
in the same model as the EPoll / KQueue subclasses in tornado itself.

I believe the API used by tornado 2.x is unchanged, so old apps shouldn't have any issues,
but I haven't tested enough to be certain. 

closes #355