Walk trough source-dir tree

[ghost]

It would be nice that multiple application can write in their own directories (with their own permission).

With a tree like that, apps can write their own separately whereas we need only one instance of beamium.

sources-dir/:
drwxrwx---  beamium  beamium  4096  .
drwxrwx---  user1    beamium  4096  app1
drwxrwx---  user2    beamium  4096  app2

Beamium should (maybe with a new config) read all sub folders of the sources-dir.

[StevenLeRoux]

As a transition I would keep reading from 'source_dir' in addition to directories so that it won't break compliancy with current deployments.