Add subject parameter for user defined subject line in emails
Ensure Emails are RFC2047 and RFC2822 Compliant

Tested with SendGrid + Google Mail for Business. Without this fix
emails sent to Google Mail will bounce with response: Reason: 550 5.7.1
[0.0.0.0 11] Our system has detected that this message is not RFC 2822
compliant. To reduce the amount of spam sent to Gmail, this message has
been blocked. Please review RFC 2822 specifications for more
information.
Update test for Base64 Encoding and Additional Headers
Remove RFC2047, Rcpt already does it
Unneeded imports
Appropriate subject encoding
Updated to pass ctest and Travis building
Merge branch 'dev' into smtp-output-subject

Conflicts:
	docs/source/configuration.rst
Reset the message string every time a new pack is processed
Merge pull request #684 from mozilla-services/smtp-output-subject

Smtp output subject