Add Platform Email Service endpoints and responses

[eporama]

A new feature of Acquia Cloud platform has been released: Platform Email service.
This is a new feature that incorporates some new API responses/actions to view/control the service.

It would be good to incorporate the new endpoints into the SDK

• http://cloudapi-docs.acquia.com/#/Applications/getApplicationEmailDomains
• http://cloudapi-docs.acquia.com/#/Applications/postApplicationAssociateEmailDomain
• http://cloudapi-docs.acquia.com/#/Applications/postApplicationDisassociateEmailDomain
• http://cloudapi-docs.acquia.com/#/Environments/postEnvironmentEmailDisable
• http://cloudapi-docs.acquia.com/#/Environments/postEnvironmentEmailEnable
• http://cloudapi-docs.acquia.com/#/Email/getEmailStatus
• http://cloudapi-docs.acquia.com/#/Subscriptions/getSubscriptionDomainRegistrations
• http://cloudapi-docs.acquia.com/#/Subscriptions/postSubscriptionDomainRegistration
• http://cloudapi-docs.acquia.com/#/Subscriptions/getSubscriptionDomainRegistration
• http://cloudapi-docs.acquia.com/#/Subscriptions/deleteSubscriptionDomainRegistration
• http://cloudapi-docs.acquia.com/#/Subscriptions/postSubscriptionVerifyDomainRegistration

Since the platform email service touches Subscriptions (registering domains), Applications (associating domains), and Environments (enabling the service), I'm not sure if it would be preferable to have "Emails" as an Endpoint/Response set or simply fold them into the existing objects.