Kristjan/get ready queue

[kristjanvalur]

This pull request adds introspection methods to uvloop.Loop and uvloop.loop.Handle.

• Loop.get_ready_queue() exposes the internal Deque, which is available as _ready on an asyncio loop object.
• Handle.get_callback() returns the registered python callback and args. for asyncio this information can be gleaned from Handle._callback

This allows uvloop to work with applications which examine and manipulate the ready queue of runnable tasks, such as https://github.com/kristjanvalur/py-asynkit. A branch which uses these features is available at https://github.com/kristjanvalur/py-asynkit/tree/dev/uvloop

An alternative implementation would emulate these same internal attributes as asyncio has, but IMHO those should also be better exposed by accessor methods.