Pull requests from #129 to #203

Changes since v1.6.0 include pull requests from #129 to #203
Includes the following notable changes

• More strict HELO, EHLO, MAIL and RCPT command parsing
• Code clean-ups, style fixes, etc
• Fix EOF when parsing the envelope headers #142
• Fixes issue #148 - use the validate processor for validation
• FreeBSD compatibility
• SQL Statement configurable for different SQL databases, not just MySQL
• add able to configure DB connection limits in sql backend
• bug fix: server-log was incorrectly pointing to the main-log #180
• Support for TLS 1.3
• Issues #195 to #201 raised by @issuefiler, mostly fixing edge cases with handling IP address-literals and other inconsistencies related to rfc5321

The major change since 1.5.1 included the introduction of a modular backend system, quite a lot of refactoring & maintenance happening since then.

Since v1.6.0, has moved from Glide to Dep dependency management tool.

Since 1.5.1, the project has developed an extensive set of automated tests. Code quality improved considerably as a result.

Latest release - before the addition of modular backends

Includes up to release noted in README

1.5.1 - 4nd Nov 2016

Small optimizations to the way email is saved