Add Compass API for Camera

[emackey]

@bagnell can you add a compass API, so that @macoda (Ravi) can hook it up to his new Navigation Widget in time for GSoC finals? I think it would basically be "get" and "set" functions that would give their best effort to control the heading angle on the spot on the Earth that the "rotate" function applies to. Maybe "get" returns undefined when no such place is available.

/cc #1060