Added a callback for (re-)initialization

Signed-off-by: Joern Bernhardt <jb@campudus.com>
Added onInit callback, updated docs for callbacks

Signed-off-by: Joern Bernhardt <jb@campudus.com>
Merge branch 'on-reinit-callback' of https://github.com/campudus/slick into campudus-on-reinit-callback

Conflicts:
	README.markdown
	index.html
minification