This commit adds support for Dyn, specifically Dyn Managed DNS. Makes
use of the Dyn REST API [1] and requires 3 environment variables for
credentails:
DYN_CUSTOMER_NAME
DYN_USER_NAME
DYN_PASSWORD
The Dyn DNS API requires a session to be established first using the
credentials above, After creating a session an api token is used for
subsequent requests.
The unit test performs a "live test" and currently requires a valid Dyn
account and domain.
[1] https://help.dyn.com/rest/
