Add context parameter to callback ...

Added a "context" parameter to callbacks. You can pass in the context
when the event is created and it will be sent back to callback when
called. The context is a void *, so you can cast it to whatever you like. 

If you don't have any context data you want to pass in (let's
say you're using separate callbacks for each timer), you can just
pass in 0 and ignore it in the callback.