Add packaging for PyPI distribution

* Add setup.py
* Rename module directory to django_akamai for consistency
* Avoid relative imports
* Whitespace cleanup
xmllint format for ccuapi-axis.wsdl
Add Akamai documentation pointer to README.rst
Purge.add(): better handling of querysets, unknown data types

* Use isinstance(…, QuerySet) to support QuerySet subclasses.

* Raise TypeError() when add() receives None or an unknown type for urls
Minor PEP-8 tweaks