Make Dired try to guess a default target directory for file operations.

"If there is a Dired buffer displayed in the next window, use its
current directory, instead of this Dired buffer's current directory."
Update MELPA URL.
Update Marmalade URL.
Remove obsolete fix for reversing package requirements.
[Refactor] Make definition of `openwith-associations` more readable by
using `openwith-make-extension-regexp` to specify extension regexps.