Merge remote-tracking branch 'upstream/master'
Vaslty improved scrolling and thumbnail loading performance

Made "loadImage:Filetype:" method use a background thread to load images greatly reducing jitters when scrolling. Also slightly reduced the thumbnail resolutions and added logic to reduce iPad thumbnail resolution independently of non-iPad. Switched the image fade transition to linear which allowed the transition duration to be reduced while still producing roughly the same visual effect.
Further reduction in fade duration for non-iPad

It looks less muddy when you scroll quickly
Scale thumbnails according to presence of Retina display.

This improves the visual look on iPhone 4/5 & Retina iPads without
taking up extra resources on non-Retina devices like the older
iPhones and iPad Mini.
Merge branch 'scrollImageFade'