add array ability to imageBackgroundBorderWidth

Use cases for extra border on 1-2 sides:
* on top for image title
* on bottom for caption
* on one side for menu
* both sides for arrows (previous/advance, eg slider)

Usage:
backwards compatible (if only single value, will coerce into array of same values)
`imageBackgroundBorderWidth` may accept array with the values [top, right, bottom, left] (order mirrors CSS)

Technical:
I don't know CoffeeScript & it is 4am so please forgive me for mistakes ;^)
Built first hacking JS directly & works; let me know if you want to see that file to compare.
swapped variable from `@options.imageBackgroundBorderWidth` to `@imgBgBorder[]`