Add test for `Proximity API`

* W3C Specification: http://www.w3.org/TR/proximity/
* Test page: [click me!](http://jsbin.com/rimiwove/2/)
* Test page in action (using the latest [Firefox Beta for Android](https://play.google.com/store/apps/details?id=org.mozilla.firefox_beta)): 

   <img src="https://cloud.githubusercontent.com/assets/1223565/3279751/f1a5e71a-f414-11e3-84c3-64bcb8445c33.gif" width=230 height=400>

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<del>There is one detail that is currently included in the [MDN link](https://developer.mozilla.org/en-US/docs/Web/API/Proximity_Events), namely:</del>
 
>   <del>"Obviously, the API requires the device to have a proximity sensor, which are mostly available only on mobile devices. Devices without such a sensor may support those events but will never fire them."</del>

<del>that I think is important, and which I don't know if should be included somewhere inline or not. (Cc: @patrickkettner)
</del>