[API] introduced virtual file system

The concrete details are now hidden on how to access the file system.

TODO: There are still parts (SPath) and other classes that have to
be correctly adapted.

Change-Id: I5d552b89fd40ae4c3be8a9e03060f569ff5813c0
Reviewed-on: http://saros-build.imp.fu-berlin.de/gerrit/1415
Tested-by: Jenkins CI
Reviewed-by: Stefan Rossbach <srossbach@arcor.de>