Add separate API for adding/removing watches

[hannobraun]

Add a separate Watches struct that provides add and remove methods to replace Inotify::add_watch/Inotify::rm_watch. It should be possible to get an instance of this struct from Inotify and EventStream (for example via watches methods on each).

I think there should be no harm in making Watches completely independent (i.e. no need to borrow Inotify or EventStream) and make it Clone.

See #112 (comment) and #112 (comment) for more context.