decode backgrounds at runtime
add OnLowMemory method and clear cache in it
Merge commit '36eed1f0ed6f50c460cb5c651b31da002dae122c' into unstable

* commit '36eed1f0ed6f50c460cb5c651b31da002dae122c':
  add OnLowMemory method and clear cache in it
load backgrounds with ImageLoader and preserve AR by cropping
fix maps