Improve formatting with 'go fmt' as stated in CONTRIBUTING.md

As 'go fmt' doesn't support verifying files in multiple directories,
it's probably a good idea to run it on all '*.go' files from time to
time with something like this:

  find . -name "*.go" | xargs dirname | sort -u | xargs -n 1 echo go fmt
Merge pull request #1690 from jbbarth/code-formatting

go fmt