go.blog/cmd/blog: use playground package from go.tools
go.blog/cmd/blog: use playground package from go.tools

R=golang-dev, dsymonds
https://codereview.appspot.com/11760043

d0f/go-zh.blog,webfd/go-zh.blog,d0f/go-zh.blog,bryanxu/go-zh.blog,bryanxu/go-zh.blog,webfd/go-zh.blog