parse_event now distinguishes between invalid and incomplete input

When parsing an event, if input is expected and not available, parse_event will return an Error with ErrorKind WouldBlock. All other error conditions are unaffected.

I'm making a telnet frontend for termion, and this distinction will let me parse in a non-blocking fashion.

Merge conflicts must be resolved.

If you still want to merge this code, fix the conflicts and talk with us in the Matrix chat.