Added "sparse" option for big-size grid
Added "sparse" option for big-size grid

Gridster becomes very laggy, when number of rows and columns is big
(e.g. 24x100, 24 columns are required for precise resizing). With
"sparse" option enabled, it won't check all cells one by one, and will
just go through all existing items.
In my case, there was a grid with 20-25 items, size - 24x100 (default
size was 6h x 12w, i.e. 2 "columns"). Item dragging from left column to
right caused browser freezing for several seconds. With "sparse:true" it
isn't noticeable.

Serenytics/angular-gridster,Serenytics/angular-gridster