set minimal __all__ in package __init__

Also added missing license in file header.
moved CloubedException in a separate file
raise exception if storage volume not found

A CloubedException is now raised when the storage volume of a domain could not
be found in the previously declared storage volumes list in YAML file.
slight update of setup.py

Added missing license in header and new metadata.
updated installation instruction in README

Now use Python distutils for installation of cloubed package.
updated import statement in examples

When Cloubed package is installed using Python distutils (now the documented
default), the import statement should first look through package directory.