3.9.1-RC2

• remove Mig Base64 because original code was complex and had no unit tests, moreover Java 8 Base64 is faster (and I'm happy to require Java 8)
• make stateless methods static
• add full unmocked StartTLS test
• improve coverage by chaining constructor in SMTPServer
• improve coverage of EmailUtils and remove unused method from EmailUtils
• cleanup - remove public from interface methods, set fields as private and final where appropriate
• improve test coverage a bit