limit the allowed paths in localstorage

this replaces the earlier way to limit the exported paths
the earlier did:
- specify a prefix in the config
- check if the prefix is in the path
- prepend the prefix to all paths
- if the path was outside the prefix, return prefix as path

the new method does:
- specify a list of prefixes to export
- check if one of the prefixes match with path
- if yes, keep path
- if not, raise notfoundexception

this has advantages:
- multiple paths can be exported
- the path is not changed unexpectedly
disallow moving up and down the path with .. and .

Before using the path, compare it with the cleaned-up
version of itself (given by os.path.normpath). It it's
different, so eitehr there's a ../ or /./ or // (double slash),
deny access and throw a 400 bad request.

The storage layer gives a MalformedPathException, the cdmi layer
hands this to the client as 400.
Merge pull request #48 from mhellmic/limitlocalpaths

limit the allowed paths in localstorage