Add golang-ci linting, tidy up lint findings.

[cpu]

This PR adds golangci-lint to ct-woodpecker's Travis CI process with a customized config. There are a couple exclude patterns added for issues that seem to me like false positives or are known things that need to be done but have a tracking issue already.

This PR also updates the repo with the same CODE_OF_CONDUCT.md used in Boulder that points to the community forum's copy of the code of conduct.

I broke API compatibility renaming monitor.MonitorOptions to monitor.Options so this should probably invoke a major version tagged release after being merged.

[cpu]

Though it passes locally I've having some trouble getting golangci-lint to pass in Travis. The errors seem related to dependency resolution. I will dig into this when I get a chance.

[cpu]

As of e6ace5e CI is passing again. I had to add a go mod download before the golangci-lint invocation to fix the dependency resolution errors I was seeing. This seems wrong and slows the build down a little bit. I think the best thing to do is merge as-is and then track solving this as a follow-up.

[coveralls]

Pull Request Test Coverage Report for Build 637

• 13 of 14 (92.86%) changed or added relevant lines in 5 files are covered.
• No unchanged relevant lines lost coverage.
• Overall coverage remained the same at 68.245%

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Changes Missing Coverage	Covered Lines	Changed/Added Lines	%
monitor/cert_submitter.go	5	6	83.33%
<!--	Total:	13	14	92.86%	-->

Totals
Change from base Build 617:	0.0%
Covered Lines:	836
Relevant Lines:	1225

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💛 - Coveralls