Add slow/fast network connection api to prevent retina images on slow connection

This may be good idea to change state of this fast/slow based on how long a couple of last images were downloaded
 but just for the start I think that it's good enough to just give user script way to ignore retina
 when it somehow detects slow connection or  maybe expose UI that would set this state based on user preferences.

On slow mobile network it is better to load crappy images and load them than loading forever good ones.