Allow FS events if not on windows but polling not set to false

When `opts.usePolling` was null, but you were on a `canUseFsEvents` platform, `opts.usePolling` would get set to true, thereby negating `canUseFsEvents`. I feel this makes the defaults a bit more sane.
Merge pull request #134 from wraithan/patch-1

Allow FS events if not on windows but polling not set to false