Add put params like on mandrill adapter

[whodidthis]

For tracking and maybe other stuff on Postmark.

[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/9.

[pablo-co]

@whodidthis thanks, this looks great! I'll merge it in.