Change it to be compatible with Twitter Bootstrap

I change the code to instead of specify device, specify a Twitter Bootstrap Media Query
http://getbootstrap.com/css/#grid

It is better approach by diferent points: 
- Isn't necessary match about isSmartDevice().. (a list that need maintenance and not cover all the devices),
- Also work for a window resize (not necessarily a device)

TODO:
- The watch to recreate the element if the size change isn't working
https://github.com/lavinjj/angular-responsive/issues/6