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.
The new one would look something like this:
```
MakeEnvUser(c *gc.C, params *EnvUserParams)
if params == nil {
  params = EnvUserParams{}
}

etc...
```