add resize_widget_dimensions to change widget dimensions after gridster initialization

(this pull request is based on the discussion of https://github.com/ducksboard/gridster.js/pull/77 and the code of https://gist.github.com/OwlyCode/6421823 with small changes, so most of the credits go to https://github.com/OwlyCode)

This will add resize_widget_dimensions() to gridster.js which allows to resize the widget dimensions after initialization:
```javascript
// initialize gridster with 200x200px widgets using a 5px margin
var gridster = $('.gridster ul').gridster({
    widget_base_dimensions: [200, 200],
    widget_margins: [5, 5]
}).data('gridster');

// resize the widgets to 300x300px and 10px margin
gridster.resize_widget_dimensions({
    widget_base_dimensions: [300, 300],
    widget_margins: [10, 10]
});
```