Add identity provider interface and agent id provider implementation
Valid login test for agent identity provider.
Merge pull request #1 from tasdomas/idprov

Valid login test for agent identity provider.
Add failing tests
Style fixes.
Merge branch 'master' into id-prov
Add user identity provider
Refactor again...
Merge branch 'master' of https://github.com/juju/juju into id-prov

Conflicts:
	state/apiserver/admin.go
Merge pull request #346 from waigani/id-prov

Identity Provider

Add an identity provider interface to state and an implementation for agents. User, Unit and Machine tags use the AgentIdentityProvider to authenticate.