Add Heroku build constraint

This adds a `heroku` build constraint. For more on build constraints, see:

http://blog.golang.org/2013/01/the-app-engine-sdk-and-workspaces-gopath.html
http://golang.org/pkg/go/build/

For additional context, I am using this to work around http://code.google.com/p/go/issues/detail?id=4210, but it is likely generally useful.

I was unsure whether to add a note about this to the README.

Also, as a bonus, I updated the README to match current heroku toolbelt behavior.