Add CDNManager, which currently provides an account list
Implements basis of CDN and origin pull support

Supported commands thus far:
* sl cdn list
* sl cdn detail
* sl cdn load
* sl cdn purge
* sl cdn origin-list
* sl cdn origin-add
* sl cdn origin-remove
Resolve rebase conflicts and #pep8probz
CDN Initial tests and fixture
Some pep changes and logics

Add maximum constants
Minor CLI help corrections
Complete CDN tests for 100% coverage (alleged to be passing)