ESP account support

 - Support for ESP account in legacy send(...) methods
 - Refactored internals of "send" request generation to use a builder-type class

The refactored internals:
 - New SendWithUsSendRequest class opens up support for tags and inline-attachments, which were not supported
 - Offers a much more object-oriented approach to API interaction
 - Allows easy addition of future "send" parameters to the API