MKAnnotation positioning fixes
MKAnnotation positioning fixes

Replace the use of the layer's anchor point and use the centerOffset property of MKAnnotationView instead. Also, there is no need to keep a retained copy of these images; let the system handle the caching.

Signed-off-by: Blake Skinner <f2809f73afc906ecbb5ce29ea24012b18700bcc7@mit.edu>

smartcop/MIT-Mobile-for-iOS,smartcop/MIT-Mobile-for-iOS,MIT-Mobile/MIT-Mobile-for-iOS,MIT-Mobile/MIT-Mobile-for-iOS,xNUTs/MIT-Mobile-for-iOS,xNUTs/MIT-Mobile-for-iOS