add parameter STORAGE to config; configure it my way
implement first storage layer abstraction

everything should import storage.py, which in turn
imports the storage backend, depending on the STORAGE
config variable.
It can currently decide between local and irods, more
can be added by modifying storage.py.
I don't see the need for automatically discovering python
modules for now.

It fails, if no matching module could be loaded (throws
an exception).

Also common parts like the Exception definitions and the
result classes are put into storage_common.py.
This class should be imported by every storage-implementing
module.