Make Environment super-command

Move set/get/unset environment commands into it to start.

Refactored many tests.
 * apiserver tests now are tested directly not through the client
 * api client tests added for connectivity tests
 * cmd/juju/environment commands mock out api completely
 * cmd/juju tests have one end to end test for each command.

(Review request: http://reviews.vapour.ws/r/609/)