Added new feature: Dinamic Size

# Dinamic Size

Think about many users using creating your own slider.
Each user uploads different images, so, we need to calculate size dinamically, not only the predefined size (800x300).

The predefined sizes still works using the css classes: "box_skitter_large", "box_skitter_medium", "box_skitter_small".

## Steps:
1. Add option "dinamic_size"
` $('.box_skitter').skitter({dinamic_size: true}); `
2. Create HTML with only "box_skitter" class:
` <div class="box_skitter"></div> `