Improved the "Files" Markdown extension:
- added support for nested folder structures
- url decode file names

Squashed commit of the following:

commit 9e7ea0375bc4e4268d5a0752ee1b2e0c860457ca
Author: Andreas Koch <andyk7@gmail.com>
Date:   Sun Nov 17 15:05:09 2013 +0100

    Files Extension: Link files

commit c21a0402088fafcf794305997bc69cf5695bde37
Author: Andreas Koch <andyk7@gmail.com>
Date:   Sun Nov 17 14:50:55 2013 +0100

    Files Extension: Code cleanup

commit b1d55f97760faab6ca9ae64caf224abc9e4831f3
Author: Andreas Koch <andyk7@gmail.com>
Date:   Sun Nov 17 14:37:43 2013 +0100

    Files Extension: URL Decode file names.

commit 047ff9f31dabc41283ebe7f4d7f0549225ab263b
Author: Andreas Koch <andyk7@gmail.com>
Date:   Sun Nov 17 14:27:06 2013 +0100

    Improved the file link rendering. Don't print the name of the
    "files"-folder.

commit 7fd2e74d19f8d0fda83453975b76a92e9f7430db
Author: Andreas Koch <andyk7@gmail.com>
Date:   Sun Nov 17 12:17:01 2013 +0100

    A somewhat working version of the files extension

commit faf35cb43178bf09181e1e6e6db81e0e18fa41a1
Author: Andreas Koch <andyk7@gmail.com>
Date:   Thu Oct 10 23:02:20 2013 +0200

    File preview refactoring: First draft
Merge branch 'dev' of github.com:andreaskoch/allmark into dev