state/apiserver/usermanager: fix potential panic
Merge branch 'master' of https://github.com/juju/juju into 165-fix-possible-panic-in-usermanager

Conflicts:
	state/apiserver/usermanager/usermanager_test.go
fix merge conflict
Merge pull request #436 from davecheney/165-fix-possible-panic-in-usermanager

state/apiserver/usermanager: fix potential panic

This PR fixes a potential panic in the usermanager which would occur if an api request to change a password arrived authenticated with an entity which was _not_ a `state.User`.

In that circumstance, `getLoggedInUser` would return a `nil`, which was not properly handled by the caller and would result in a panic.

As all callers of the `getLoggedInUser` function were actually after the `state.Entity`'s tag, I changed the method to return the tag.