Readme examples

Hi, 

Some thoughts about the README while I was using tornado_pyvow for the first time.

Add example using IOLoop.instance() singleton - may not an ideal example but maybe useful to know why the tornado AsyncHTTPClient isn't working in the vows.

Use lambda instead of partial - I assumed it was functools partial, but it could been some tornado_pyvows thing. I think lambda is more clear, or show the imports.

I'm getting the hang of it though,

Cheers.