Better handle `to` from Mailgun

* First, try to get full address from To param
* Fall back to header
* Default to recipient.
Better error for missing EmailProcessor

When a NameError is raised because both Griddler.config.processor_class
has not been set and EmailProcessor is undefined, provide a more
informative message than `uninitialized constant EmailProcessor (NameError)`.

* Eager load files in Rails 4

  Since the change in Rails 4.* to lazy load files, `const_defined?` did
  not return true for an EmailProcessor before it had been referenced.
  We have to eager load in the test environment to circumvent this.