Support managing dotMailer data fields

I'm opening this to get feedback on the structure of the gem, how we are interacting with the API, and how the tests look.

This pull request doesn't include the initial commit (1c89be0) as that is what I am "pulling into" but feedback on that commit is also welcome.

I have not added tests against the live API, just stubbed out how I expect it to behave. Thoughts on this also welcome :wink: