Add. Support to `socket_action` interface.
Fix. Init socket callback with nil.
Fix. `multi.setopt` supports socket callback.
Change. `socket_acttion` without args means timeout action.
Change. cURLv3 multi supports options in callback.
Add. `multi-uv` example.
Update doc.
Fix. Cleanup socket callback.
Merge pull request #46 from moteus/master

Support `socket_action` interface.