add signer and DKIM configuration
add Signer service
add DKIM signer
fix configuration break
update documentation for DKIM signature
remove duplicate code in configuration ext
fix unit test of MessageFactory
Fix typo in doc for DKIM signature
Merge branch 'signed_header'

Fixed Conflicts:
	DependencyInjection/LexikMailerExtension.php
	Resources/config/services.xml