2.0.0 Release

This are still a work in progress and there's still some [open issues](https://github.com/guard/listen/issues?direction=desc&milestone=2&page=1&sort=comments&state=open) to manage, more that list of feature to add:

- [ ] `force_adapter` option
- [ ] Raise/Warns if OS-specific adapter doesn't work? (no more automatic fallback polling)
- [ ] Update BSD adapter
- [ ] Update Windows adapter
- [ ] Migrate CHANGELOG.md to GitHub releases