Adding support for creating and deleting affinity groups

This pull request adds support for more affinity group API calls in the SDK. It is a replacement for the https://github.com/WindowsAzure/azure-sdk-for-node/pull/539 PR and is now complete with unit tests and based on the dev branch.

@glennblock @andrerod Sorry for messing up the original PR with the wrong branch. I hope I got everything right this time! :-)