Add a package docstring to bridgedb.email.
Split up Sphinx source files pertaining to email documentation.

 * CHANGE Sphinx source files to mirrow layout of the new email module.
Move email autoresponder code to separate module.

 * ADD new bridgedb.email.autoresponder module.
 * MOVE b.e.server.createResponseBody() → b.e.autoresponder.createResponseBody()
 * MOVE b.e.server.generateResponse() → b.e.autoresponder.generateResponse()
 * MOVE b.e.server.MailResponse → b.e.autoresponder.EmailResponse
 * ADD new b.e.autoresponder.SMTPAutoresponder class, created mostly
   from pieces of b.e.server.MailMessage, but made more compatible with
   Twisted's existing SMTP server/client architecture.