Extract Mandrill adapter

[calebhearth]

Due to the fact that thoughtbot don't use email POST services other than SendGrid, we've run into a problem with the other service adapters: they don't always work correctly, and they add a maintenance burden to the main project.

Because of this, we've determined that we want to extract all adapters from the Griddler gem into subgems, and direct users to add the specific gem for the service they use.

We'd love to get someone involved in that adapter gem who would be able to provide more knowledgeable maintenance of the adapter. I'm including mentions to everyone who's contributed to the adapter, as well as to the MailChimp organization and the few members I saw on the organization page.

If any of you are interested, please let me know.

Subtask of #124

History of file (to determine churn and approximate workload) Issues related to Mandrill

Previous contributors to the adapter: @wingrunr21 @pbougie @lagartoflojo and @r38y MailChimp: @mailchimp @amro @stephenmartin @stevenosloan

[calebhearth]

Whoops, wrong set of people for the contributors. The actual people are: @wingrunr21 @pbougie @lagartoflojo and @r38y

[wingrunr21]

I'd be willing to maintain the Mandrill adapter

[calebhearth]

Great. We're going to start talking about the logistics over in #124

[calebhearth]

This portion of the README has been removed, but is something you'll want in yours:

Using Griddler with Mandrill
----------------------------

When adding a webhook in their administration panel, Mandrill will issue a HEAD
request to check if the webhook is valid (see [Adding Routes]).  If the HEAD
request fails, Mandrill will not allow you to add the webhook.  Since Griddler
is only configured to handle POST requests, you will not be able to add the
webhook as-is. To solve this, add a temporary route to your application that can
handle the HEAD request:

```ruby
# config/routes.rb
get "/email_processor", to: proc { [200, {}, ["OK"]] }, as: "mandrill_head_test_request"

Once you have correctly configured Mandrill, you can go ahead and delete this code.

[calebhearth]

Resolved by #152