Clean up bus initialization

* We now explicitly initialize the bus in activity, or in
beforeEach for tests. This is more predictable that initializing
whenever the bus module is loaded. Also it will allow to write
tests that uses a mock implementation of the bus socket.
* Environment setup is factored out to its own module. It will
allow other modules which needs to get values from the environment
to do so.
* The environment is taken from a single window.sugar.environment
object.