Add API func ShareEnvironment to client and server API:

ShareEnvironment shares the current environment with the given user.
Under the hood, it adds an environment user to state.
Merge branch 'master' of https://github.com/juju/juju into share-env-take2
Merge branch 'master' of https://github.com/juju/juju into share-env-take2
Merge pull request #645 from waigani/share-env-take2

New client & server API func ShareEnvironment

ShareEnvironment shares the current environment with the given user.
Under the hood, it adds an environment user to state.