Introduces version string in log message

Adds a version string to enable the user to distinguish between
multiple Prosody-Filer versions. The version information is "baked in"
via the linker in build.sh

Fixes ThomasLeister#14: Prevents directory listing in subdirectories

This commit prevents directory listings in every directory level,
not just on root level ("upload/").

In earlier versions, an attacker could get a list of past uploads
if he ever received a valid file download path, removed the file name and
descendet in parent directories.

Adds CORS headers

Merge pull request ThomasLeister#2 from xamanu/master

Avoid serving overview of main directory

Adds support for Gajims url_image_preview plugin by sending valid con…

…tent-type headers

RC1 try ThomasLeister#3