Support locking of directory entries

[a-sully]

Currently, a lock may only be taken on a file entry. This should be extended to support locking directories, too.

This is one of the proposed changes to support #34, but it's also needed to properly support operations which modify a directory (such as remove() #9, removeEntry(), and move() #10 of directories)

Operations which modify a directory should require taking an exclusive lock on the directory, which should only succeed if no contained file is locked