Improve code.google.com presenter to fetch changes when needed.

If a given commit is not already available in remote vcsstore, try to
clone or update the repo in vcsstore and try again.
Include debug output.
Remove debug output.
Merge pull request #22 from shurcooL/update-remote-repo-when-out-of-date

Improve code.google.com presenter to fetch changes when needed.
Work on porting frontend JavaScript code to Go.
Fix script.go having non-main package.

That resulted in main() not being executed. See gopherjs/gopherjs#75
and go-on/gopherjslib#3 for relevant context.
Remove unnecessary //gopherjs:blocking comment.
Add js build tag.
Do not hardcode server address for frontend POST request.
Serve compiled JavaScript with .js extension to avoid confusion.