Fixes for #292

Changes:

- Refactored `isCdnEnabled` method from abstract base class `AbstractContainer` into derived classes `Container` and `CDNContainer` to prefer polymorphism over use of `instanceof` in abstract base class.
- Fixed logic in `isCdnEnabled` method of `Container` class.

**CAUTION!**  I tested these changes against my test cloud account and they seem to fix the issue reported in #292 and do the right thing. Still, one unit test fails and I'm still trying to figure out why. I wanted to submit the PR anyway to get some movement on this issue.

Also, I'm not sure why the changes to .travis.yml, Keypair.md and Object.md are being pulled in here. As far as I can tell they already exist in the working branch of the rackspace repo.