Bug 1088905 - Support with syntax when setting marionette's context, r=ato

Usage:
# content scope
with marionette.using_context(marionette.CONTEXT_CHROME):
    # chrome scope
    ... do stuff ...
# content scope

--HG--
extra : rebase_source : 1a823ec0a92e14b0f49df961e155a64917efe994