This changes the prototype of uv_run() from:
int uv_run(uv_loop_t* loop);
To:
int uv_run(uv_loop_t* loop, uv_run_mode mode);
Where `mode` is UV_RUN_DEFAULT, UV_RUN_ONCE or UV_RUN_NOWAIT.
Fixes#683.
Allows for running the event loop in 3 modes:
* default: loop runs until the refcount drops to zero
* once: poll for events only once and block until one is handled
* nowait: poll for events only once but don't block if there are
no pending events
This can be used in conjuction with uv_run_once() to poll in one thread and run
the event loop's event callbacks in another.
Useful for embedding libuv's event loop in another event loop.
