Prevent overlapping the buttons

Example:
```(javascript)
$.prompt('<img src="http://upload.wikimedia.org/wikipedia/commons/a/a9/Example.jpg" style="float:right" />text');
```
Prevent shifting buttons

Example:
```(javascript)
$.prompt('text',{buttons:{'Button 1':true, 'Button 2 long':true, 'Button 3 very loooooooooooooooooooong':true}});
```
Merge pull request #48 from Zalmoxisus/dev

Prevent overlapping the buttons