add and use localstorage.py as storage

the simplest implementation possible and supports only read()
add stat and ls function to localstorage.py
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.

fix import of storage in cdmi.py

add the storage config parameter
add config for the gevent server
disable sending the content-length for files

not sending the content-length enables the frontend
to use transfer-encoding chunked to stream the request.

with twistd, the memory consumption stays high,
with gevent, it is reduced a lot.
correct the import for the sysloghandler