add IOLoop.time

the method is added in current tornado master, in the 3.0 refactor.

I will update IOLoop for real when tornado 3.0 is closer to release,
but this seems to be the main issue when using pyzmq IOLoop with tornado-dev.
Merge pull request #314 from minrk/ioloop.time

add IOLoop.time

the method is added in current tornado master, in the 3.0 refactor.

I will update IOLoop for real when tornado 3.0 is closer to release, but this seems to be the main issue when using pyzmq IOLoop with tornado-dev.