Update State.AddUser to validate username

Use names.IsValidUserName to validate the user and creator string
passed in to State.AddUser.
Move state/api and state/apiserver to top level

This is a follow up to #653 that finishes the general code migration.
gofmt
Merge pull request #655 from davecheney/191-move-state-api-apiserver-to-top-level

Move state/api and state/apiserver to top level

This is a follow up to #653 that finishes the general code migration.
Merge pull request #654 from waigani/AddUser-IsValidUserName

Update State.AddUser to validate username

Use names.IsValidUserName to validate the user string
passed in to State.AddUser.
Remove environs/tools.FindInstanceTools