Fixes Windows tablet detection.

Using the example provided in the repo and the latest version of device.js (0.1.59) Windows Phone 8 devices are incorrectly detected as "tablet" rather than "mobile". 

I've confirmed this issue on an actual device as well as in the emulator (availble in the [Visual Studio 2013 Express for Windows](http://www.visualstudio.com/downloads/download-visual-studio-vs#d-express-windows-8) download).

This fix updates the `windowsTablet()` method with an additional check to ensure the device is not a phone. The `windowsTablet()` function now looks like this:

````coffeescript
device.windowsTablet = ->
  device.windows() and (_find('touch') and not device.windowsPhone())
```