Specified media type in media queries, fixes a bug in safari when printing

Fixes a bug in safari when printing content.

It seems some styles dedicated to screen applies in safari when printing.

Here is how to reproduce the bug:

1) Create a simple HTML document like this one: http://ghusse.com/bootstrap-bug/
2) Print it with safari, chrome, Firefox, IE

In Chrome, the result is similar to the result on screen:

![chrome](https://cloud.githubusercontent.com/assets/527245/4023488/97e1b47e-2b92-11e4-867e-d7bfee2de71c.png)

In Safari, something weird happens:
* The page is truncated on the right
* Font sizes are smaller

![bug-safari](https://cloud.githubusercontent.com/assets/527245/4023490/a6f66374-2b92-11e4-8806-0d88a26e52c8.png)

With this fix, everything works great in safari too

![safari-fixed](https://cloud.githubusercontent.com/assets/527245/4023491/bd926182-2b92-11e4-9988-c360f498e247.png)

If you want to test, here is the buggy version: http://ghusse.com/bootstrap-bug/
And here the fixed one: http://ghusse.com/bootstrap-bug/nobug.html

The fix consists to specify the media type (screen) on each media query specifying a min-width.