Added support for setting min and max sizes of individual gridster-items
In addition this patch also prevents resizing components to be wider than the container.columns size (ie. item.sizeX must now remain <= gridster.columns)

Available options are
 - minSizeX
 - minSizeY
 - maxSizeX
 - maxSizeY

All options are optional and you can set them in the same way you currently set sizeX, sizeY, col and row options.

For example to restrict resizing of an item to 2 rows high by 2-4 columns wide do the following:

components = [
    … ,
    {
        grid: {
            minSizeX: 2,
            sizeX: 3,
            maxSizeX: 4,
            minSizeY: 2,
            sizeY: 2,
            maxSizeY: 2
        }
    } , …
]

or to restrict only the width to 3 columns:

components = [
    … ,
    {
        grid: {
            minSizeX: 3,
            sizeX: 4,
            sizeY: 2
        }
    } , …
]