Added support for train bearing from the API

* Include bearing (degrees clockwise from north) as reported by the MBTA
  API
* Apply corresponding rotation to map annotations so markers point in
  the actual direction reported for the trains

Bug in this version: by rotating the entire annotation view (by affine
transform), I inadvertently also rotated the callout bubbles.  First
attempt at a fix was to rotate the UIImage instead of the annotation
view; that worked but it broke the animation, as the change of image
from one rotation to another could not be animated.

Next fix attempt will be to put the image in a subview which can be
rotated independently of the annotation.
Added missing optional tests when updating
Removed debug output
Moved marker into subview for independent rotation

Fixes bug in initial version of train-bearing display support where the
callout bubble was rotated along with the view.  Now the image is
contained in a UIImageView subview of the main annotation view, which
is rotated relative to the annotation view, which remains itself
unrotated.

This update also adjusts the center offset for the annotations to move
them orthogonally to the direction of travel, in effect placing them
on the right side of the track.

Bug: marker rotation still doesn't take map rotation into effect.