task/ta-npi-022-08 -- integrate  RestEmailer into loader.py

nparcel/emailer.py
* send() method signature now supports mime-based message string via
  new argument, mime_message
* refactored send() method so that it can send a MIME message inline
  or generate a message from a provided message string

nparcel/loader.py
* __init__() method signature update to support email_api kwargs
  attribute
* refactor the send_email() method to support the new RestEmailer()
  class

nparcel/loaderdaemon.py
* update _start() method so that Loader() call supports email_api kwargs

nparcel/restemailer.py
* refactor work to make production ready

nparcel/templates/email_html.t
* make embedded images base64 to support Esendex platform