Add mail package.
Add mail package.

Change-Id: I0ee5ac9503881721845003b62a5db0c867427c36

mzimmerman/appengine,skyeunknown084/appengine,skyeunknown084/appengine,flowlo/appengine,mzimmerman/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,golang/appengine,flowlo/appengine,flowlo/appengine,fuchsia-mirror/third_party-github.com-golang-appengine,mzimmerman/appengine,skyeunknown084/appengine,golang/appengine