Use window.inner width

Hello Michael,

I improved two things in the extension, the first one is that I need two clicks to see the media query, and the second one, is that if my DevTools is open I don't get a precise media query, I need to literally resize all the window.

Here is how the extension is working today:
![screen shot 2014-09-14 at 3 42 14](https://cloud.githubusercontent.com/assets/736728/4263279/50e1f6ee-3bda-11e4-82fd-d91a5b60f9da.png)

See that the width is not the true width, because my DevTools is open, it is like the `window.outerWidth` property, but I want `window.innerWidth` property.

Here, is how the extension is working with my improvements, now the extension use the `window.innerWidth` property.
![screen shot 2014-09-14 at 3 47 49](https://cloud.githubusercontent.com/assets/736728/4263287/54300c18-3bdb-11e4-8285-87d5859f3c4e.png)

You can also emulate a device with DevTools and get the media query from the emulated window.
![screen shot 2014-09-14 at 3 49 29](https://cloud.githubusercontent.com/assets/736728/4263289/61f765bc-3bdb-11e4-8691-5d14cd0f359c.png)

What you think about these improvements?