Modified the code to show the picture number instead of a checkmark.

Signed-off-by: Esteban Torres <me@estebantorr.es>
Reverted checkmark view back to being a checkmark
Reverted BSPhotoCell back to not using borderview, etc
Use array index as picture number and not the array count
Reload picture numbers when deselecting
Changed model to keep selected assets in an array instead of nsset. So we can get a correct picture number
Brought back the "checkmark" with number
Renamed BSCheckmarkView to BSNumberedSelectionView
Don't show faded cover view when in preview
Update photo number in preview
"sync" => reload sections :)
Update done button with the total number of selected assets
Fixed a bug where you always would end up with the first image after long press (the first time)
Removed assumed index for button and instead perform a check by setting enabled
Updated some comments
Merge branch 'feature/numbered_selection' into develop