Add Postmark email opens and links tracking options

[whodidthis]

Hopefully the api is fine.

Do you think the adapter should throw if TrackLinks option is not one of the possible values "None", "HtmlOnly", "TextOnly" or "HtmlAndText"?

http://developer.postmarkapp.com/developer-api-email.html

[pablo-co]

Ebert has finished reviewing this Pull Request and has found:

• 1 possible new issue (including those that may have been commented here).

You can see more details about this review at https://ebertapp.io/github/pablo-co/bamboo_postmark/pulls/8.

[pablo-co]

Thanks for your work!

To better reflect the act of tracking I would rename the tracking function to the imperative track as in giving an order.

For the link tracking I think it would be cleaner to declare them through symbols: :none, :html, :text, :html_and_text. The advantage is that we give a cleaner more abstract interface to Postmark; the drawback is that we loose transparency towards which values are valid and the equivalence between the symbols and string value they get converted to. What do you think?

[whodidthis]

Actually now that I look at the official adapters it looks like we should just provide a put_param for the additional parameters: https://github.com/thoughtbot/bamboo/blob/master/lib/bamboo/adapters/mandrill_helper.ex#L15

I can redo this pr more in line with the official ones if you want?

[pablo-co]

Sorry, the last 2 weeks have been really busy. It actually sounds like a great idea to use a put_param function instead.

[pablo-co]

Closed in favor of #9.