Add 2014 in COPYING
Major rewrite: sync with silverstripe-gallery

Notable improvements:

- use sortablefile for reordering;
- ported to Bootstrap 3;
- resize images with optional width and height.
Correct dependency to sortablefile
Name the group of fields in Settings

That group will be extended by silverstripe-gallery, so it should be
accessible from the outside in a viable way.
Cosmetic changes
doc: add Usage section header
Use custom folder for picking images

Search the first class name (i.e. the first with a 'Page' suffix) in the
object hierarchy that has a correspoding folder in ASSETS_PATH, that is
a folder with the same name but without the 'Page' suffix. This folder
will be used as custom path.

For example, if this class is `HomePage` and it is inherited from
`CarouselPage`, will be checked `assets/Home` first and
`assets/Carousel` after.

If no valid folders are found, the default folder is used.