ref: Fix gofmt, gocyclo, ineffassign and misspell warnings

[kamilogorek]

Let's make it all 100% before moving forward – https://goreportcard.com/report/github.com/getsentry/raven-go

• Fixed:
  • gofmt
  • golint
  • gocyclo
  • ineffassign
  • misspell
  • errcheck
• Added simple logger with SetDebug function
• Configured TravisCI to run golint (it'd be nice to use https://github.com/alecthomas/gometalinter instead, but it'd require us to drop support for <1.9 [which would be nice anyway, as we could use type aliases and start unifying some old interfaces to follow the spec and naming conventions from https://docs.sentry.io/development/sdk-dev/interfaces/)

[kamilogorek]

Updated.

[mitsuhiko]

I would redirect these prints through a helper function that can be silenced.

[kamilogorek]

Added. We can start logging more details using it from now on.