Wrapper for raw events

[orthecreedence]

It would be useful to be able to monitor an fd without having to wrap a bufferevent around it, in case you just wanted to be notified of new data, for instance if integrating cl-async with an existing driver.

It would be easy to wrap the event portion of libevent, but the tricky part would most likely be intelligently freeing the event. Right now, cl-async generally takes care of all freeing of resources depending on what events are processed, and this would introduce an API where the library user would have to be aware memory management, which I'm not too keen on.

Perhaps it's not a big deal though, as long as I make note of it in the docs.

[orthecreedence]

Solved by issue #61!