implemented a (draft) Marketo unsubscribe webhook handler. the handler is in draft status because we have not finalized nor tested the webhook yet.

update the do_not_email user profile attribute and handle its update to unsubscribe or subscribe a user

see https://github.com/GigaOM/legacy-pro/issues/3858
tested the webhook ajax callback, and use the webhook secret
use a more readable config var name for the webhook secret
better error-checking and use "if" instead of "switch" on a variable that only has one valid expected value currently.
call the go_marketo_post_lead_update action after sync'ing a marketo user
updated unit tests
Merge pull request #2 from wluo/github-3858

Marketo unsubscribe webhook handler