Major changes ahead

I needed to plow ahead with angular-gridster development for a project at work and I regret not being able to involve you guys. I just thought I'd share the work I have done. Let me know if you are interested in moving the project in this direction otherwise I could create a new project with your approval.

Demo: https://cdn.rawgit.com/jdewit/angular-gridster/feat/responsive/demo.html#/dashboard

Summary:
- Use interact.js instead of jquery UI for dragging/resizing
  functionality 

- Widgets can be resized with pinch/zoom on tablets

- Improved responsiveness by storing positional information for various
  screen widths on the item (ex. desktop, tablet, mobile).

- Stacking on mobile devices has been removed. You can achieve the same
  effect by setting the defaultSizeX to the number of columns for the
mobile mode. This is default.

- Added GridsterConfig constant to allow for setting your preferences
  site wide.

- Allow options set in the modes object in GridsterConfig to override
  the default options

- The markup to initialize the directive has been simplified to
```
<div gridster="items"></div>
```