handle do_not_email user attribute correctly

* do not sync users that have `do_not_email` user profile set
* when sync'ing a user triggered by setting the `do_not_email` attribute to `TRUE`, do not attemp to create a lead if it doesn't exist on Marketo already
* added an option to create or update a lead or just update the lead (do not create one)
* use array instead of stdClass when creating marketo api parameter

see https://github.com/GigaOM/legacy-pro/issues/4168