Rename future_wrap to return_future.

Allow exceptions from the initial synchronous phase to pass through
to the caller.  Add docs and tests.
Add decorator for @gen.engine-like tests.

Futures are awkward to use in stop/wait tests, and this makes it possible
to be consistent between tests and regular code.

Uses the (now-renamed) deactivate_stack_context callback
to detect when a test finishes cleanly.
Support lists of futures in @gen.engine.
Protect the tornado.gen import in testing.py for app engine.
Add a script for setting up a windows VM for testing.

This works with the images provided by http://modern.ie.