Switched to use picasso for image loading.
Cleaned up icon loading requests.
Also fixed file extension inconsistencies.

Still needed:
Move logic of getting icon URI to DirectoryScanner.
Fix APK icon loading.
Remove shouldLoadIcon() altogether.
Improved APK icon loading.
Improved apk and associated activity icons.
Renamed ThumbnailHelper.
Associated activity icons are now resolved correctly.
Removed redundant file icon preference.
Removed ThumbnailLoader in favor of Picasso & ThumbnailHelper.
Switched to UIL.
Finalized UIL implementation.
Merge branch 'develop' into bug_22_thumbs
Upped delay before loading thumbnails as it could produce slight lag when scrolling fast.
Changed hidden files default visibility -- now hidden.
Icon selection improvements for files that don't have associated apps.