Adds BW::Location.get_compass & BW::Location.get_compass_once

Returns a hash of a CLHeading object. Example usage:

BW::Location.get_compass do |heading|
  puts heading[:magnetic_heading]
end
Add test to make sure BW::Location.stop stops updating the compass heading.
Typo in get_compass spec.
Changes to readme: add get_compass documentation.
Merge pull request #348 from markrickert/location-compass

Adds BW::Location.get_compass & BW::Location.get_compass_once