Handle Dired buffers correctly

(buffer-file-name) evaluates to nil in Dired buffers: Use
default-directory (current directory) as fallback in Dired(+) buffers.
Keep wg-list in MRU order

This will also ensure that the last used workgroup will be used on emacs startup. Fixes #18.
Merge pull request #1 from juergenhoetzel/master

Handle Dired(+) buffers correctly and keep wg-list sorted (MRU)