Delete dir link support

Adds support for LinkOptions to `directory?` and `delete-dir` on Java 7.

The resulting usage looks like:
```clojure
;; As before:
(delete-dir root)

;; If you do not want to follow symbolic links
;; This deletes the link only, not the contents of its target directory
(delete-dir root LinkOption/NOFOLLOW_LINKS)
```