New EnvUser

Add:
 - the EnvUser doc
 - support for setting the last login time for the EnvUser
 - factory methods for adding the envuser

Create the Admin EnvUser at bootstrap time.
import cycle:
`./upgrades_test.go:9: import "github.com/juju/juju/testing/factory": package depends on "github.com/juju/juju/state" (import cycle)`