A new IOLoop is automatically "current" if there isn't already one.

A new IOLoop is automatically "current" if there isn't already one.

IOLoop keyword arguments throughout the library are now deprecated
in favor of IOLoop.current(). This is because the gen module
uses IOLoop.current() with no opportunity to pass in a different
instance, making it impossible in some cases to use a non-current
IOLoop with AsyncHTTPClient.

Closes #1252.