wip - Always return an array of recipients for email.to

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.
Handle multiple recipients from Cloudmailin

Per Steve at Cloudmailin, header fields are not parsed. This means that
they will simply be comma-separated.
Methods names can be nouns. Weird.
TIL merge doesn't need braces.
Less parameters
Add upgrade text to README, post_install_message
Deprecate :token option for config.to

I am deprecating the token option because with the possibility of
getting multiple recipients it becomes dangerous to operate only on the
token.

Take, for example, an email delivered to `topic.1@griddler.com` and to
`sam@looped-in-to-conversation.com`. With only tokens, the client
application would only see ['sam', 'topic.1'], which could cause
problems.

:token will remain available through the :hash option, which is now the
default option.
Remove trailing '"'