Move be_normalized_to matcher to Griddler::Testing

This allows adapter gems to take advantage of the matcher.
Remove Postmark adapter

Replace with griddler-postmark gem.

Resolves #129
Remove Cloudmailin adapter

Replace with griddler-cloudmailin gem.

Resolves #126
Remove references to adapter gems in code

* Remove dependency on adapter gems
* Change :sendgrid references to :default for email service and
  configure a fake one rather than referencing
  Griddler::Sendgrid::Adapter
* Stub out a fake "working" adapter for when it matters

Resolves #124
Update body parser regex to allow for new lines

* Add spec for and handle proper parsing of some Gmail responses
Remove unnecessary Griddler::Email#process calls

I'm not entirely sure why these were present, but the tests all pass
without the call. Probably a mistake made early on that was copied
whenever a new test was added.

Since it's not clear from this diff, #process gets the
Griddler.configuration.processor_class and sends it
Griddler.configuration.processor_method with self (the Griddler::Email
instance) as the argument.

This is the first step in moving the process logic out of
Griddler::Email and into the controller.
Move process logic out of Griddler::Email

This is controller code that shouldn't have been in a data object to
begin with.
Use instances rather than class methods

Rather than sending Griddler.configuration.processor_method to
Griddler.configuration.processor_class with a Griddler::Email as an
argument (i.e., EmailProcessor.process(email)), instantiate
Griddler.configuration.processor_class with a Griddler::Email and call
Griddler.configuration.processor_method on that object (i.e.,
EmailProcessor.new(email).process).

This encourages the behavior of separating logic into methods within an
EmailProcessor, which is better than having a single class method with
many responsibilities or passing state around between class methods.
Encapsulate Griddler.configuration calls