More file system checking functions:

- Stat#isFile
- Stat#isDirectory
- Directory.exists()
- File.exists()

doc/site/modules/io/directory.markdown

@@ -4,6 +4,11 @@ A directory on the file system.
 
 ## Static Methods
 
+### Directory.**exists**(path)
+
+Whether a directory exists at `path`. This returns `false` for files or other
+special file system entities.
+
 ### Directory.**list**(path)
 
 Lists the contents of the directory at `path`. Returns a sorted list of path