Minor cleanup.

Apply gofmt on html.go.
Apply goimports-compatible formatting on block.go (space between standard and third party imports).
Move Travis build status image in a more pleasing, common location.
Remove "Markdown pretty-printer output engine" from TODO steps; this is already done in markdownfmt.
Remove unneeded trailing whitespace in README.
Merge pull request #131 from russross/minor-cleanup

Minor cleanup.