Add {Event,Op}.Has() (#477)

Add {Event,Op}.Has() (#477)

Using:

	if event.Op&fsnotify.Create == fsnotify.Create {
	}

is overly verbose and awkward; a Has() method makes this much nicer:

	if event.Has(fsnotify.Create) {
	}

PR #76 was previously rejected; I think that HasCreate() etc. is
overkill, but a Has() method makes the library quite a bit more
convenient IMO.

I added it to both the Event and Op to avoid confusion; with this we can
change the Op field to be private in a future v2.

Fixes #107