Always return an array of recipients for email.to

**This is a work in progress - I've made assumptions about SendGrid's
format for multiple recipients and don't know what Cloudmailin's format
is at all. I have send questions to both services for clarification on
that point**

While this is a change that will break backward compatibility, it is
ultimately beneficial in that it will simplify the implementation of
`EmailProcessor`s, which can now simply loop over the recipients in
`email.to` and check to see if the address is relevant to their app.

This will also fix problems which may have been hidden when `email.to`
returns only one of potentially several addresses, in what may not have been
a valid format depending on an app's `Griddler.configuration.to` setting.

* Convert all adapters to wrap `normalized_params[:to]` in an array,
 even for a single recipient.
* Add a feature spec for the integration between adapters and
 `Griddler::Email` which ensures that the parsed address is the same
 regardless of email service.