Add Google Cloud SQL support for App Engine.

It uses Cloud SQL as a database if it runs on Google App Engine with using cloudsql DSN.

appengine.go and appenginedev.go include a [build constraint for GAE](http://blog.golang.org/the-app-engine-sdk-and-workspaces-gopath) to avoid an error outside App Engine and provide an exception. 

Using Google Cloud SQL with App Engine GO SDK is not supported yet. It is a SDK limitation.