prioritise image downloads

[aneesh-joseph-1]

If you have multiple Activities and loads of ImageViews which are rendered by Picaso, then the active Activity/ImageView is not given preference.It keeps downloading images of the first activity which has gone into background.

[JakeWharton]

We have talked about this internally. It's on our roadmap.

[dnkoutso]

Marking for 2.3.

[dnkoutso]

I do not believe this is actually the case. The first requests wil be decoded but eventually the rest will get cancelled and ignored.

I will be investigating further.

We do need some logging to ensure this does not happen though.

[JakeWharton]

#665 & #666.