Extended the demo with an example for the new horizontal bar mode.
Cleanup of the new horizontal bar functionality.

 - added class prefix to the bar class, to minimize the chance for namespace collisions
 - exposed and documented the MBBarProgressView interface
 - removed some non-crucial methods to simplify the API
 - adjusted the default values for some attributers
 - cleaned up the code style, so it matches the rest of the class
 - added proper ARC macros
 - using KVO instead of accessor overrides for UI update triggering
Fixed the determinate demo.
Migrated some property overrides to KVO on MBRoundProgressView.
Added a few pragma declarations to MBBarProgressView.