convertEvent now returns commonEvent values

This fixes adam000/Go-SDL2#8 by not returning nil.

I also added a HasEvent function, and cleaned up some of the docs.
Use unsafe.Pointer instead of C functions to convert events
Use commonEvent for quit events
Clean up EventType
Fix SDL_WINDOW_* constants to directly use C values
Rename all event types with EventType suffix
Specify WindowFlag type for all WindowFlags
Merge pull request #10 from zombiezen/eventfix

Fix event.go