change the routing and layout of cdmi storage-access functions

1. change of routing:
  a function routing everything from '/' as catch-all
  now dispatches to the cdmi function. For each http request
  (PUT,GET,DELETE at the moment) there is one route function.
  Each separates between directory and file with looking at
  path[-1] == '/'.

2. change of structure:
  all functions called by the route functions move to cdmi.py.
  Also the function wants_(cdmi|json) move there. wants_json is
  duplicated so far as it's also used by other route functions.
  This should be cleaned up and moved into a lib file.

This allows the following improvements:
  We can now match '/', which wasn't possible before
  The routes.py is much shorter and has fewer non-route functions