Support local repositories and files

By simply adding an absolute (or relative to the npackd working directory)
file path with the file:/// prefix, users and packagers are now able to use
their own repositories and packages without having to host the files on a
web server.

Per example use the URL:
file:///C:\Users\jdoe\directoy with spaces\my repository.xml
to reference the repository XML file located at:
C:\Users\jdoe\directoy with spaces\my repository.xml

This functionnality also offers npackd the possibility to have a full
offline support. Which means that npackd can rely on only local files
without requiring any internet access. Possibilities such as USB/CD
installers, personal packages and collections are now a little easier (some
little constraints are still presents).

Finally, even if would I still recommend to ban them from your file system,
spaces in file:/// URLs are also supported.
Directory tree support for <important-file> shortcuts

It is now possible to define directory trees for the <important-file>
shortcuts. Simply add \ or / into the 'title' attribute value of the
<important-file> property and let npackd automatically make/remove the
directory tree structure at the install/uninstall phase.

This integrates a non-destructive cleanup process. Which means that the
directory trees containing the shortcuts will be automatically removed only
if empty.