[proposal] Add `/browserconfig.xml` and tile images ?

[proposal] Add `/browserconfig.xml` and tile images ?



By default, starting with IE 11 on Windows 8.1, IE requests  `/browserconfig.xml` whenever a user hits the `pin site button` <img src="https://f.cloud.github.com/assets/1223565/1736715/6bd969f6-6367-11e3-8b00-3bc09f01c5f5.png" width=25 height=25> (same as Safari for iOS does with the `apple-touch-icons` when the user hits the `share button` <img src="https://f.cloud.github.com/assets/1223565/1736738/c43f0f60-6367-11e3-9431-30abd9441bf7.png" width=25 height=25> ).

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Note(s): 

* from my tests, `/browserconfig.xml` seems to be requested even if the tile images are specified in the `html` page:

  ```html
<meta name="msapplication-TileColor" content="#000"/>
<meta name="msapplication-square70x70logo" content="tiny.png"/>
<meta name="msapplication-square150x150logo" content="square.png"/>
<meta name="msapplication-wide310x150logo" content="wide.png"/>
<meta name="msapplication-square310x310logo" content="large.png"/>
```

* to specify a custom config file, use the following `meta` tag in the `<head>` portion of the page:

  ```html
<meta name="msapplication-config" content="someotherconfigfile.xml" />
```


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Adding `/browserconfig.xml` and the tile images:

* reduces the number of 404s, especially as IE 11 and Windows 8.1 get more market share
* provides a starting point for dealing and customizing Windows tiles 
* reminds users about the tile images, so that their site doesn't end up having as a tile the default IE11 logo, or (if the favicon is 32x32px or bigger and not `.ico`) the site's favicon 
* reduces the page size compare to using `<meta>` tags (the `/browserconfig.xml` isn't delivered with every page, and is only requested when the user clicks the `pin site button`)

Only two images are used instead of four, as:
    
* 558 x 558px (Large) - is used for the `Small`, `Medium`, and `Large` tiles, being automatically resized where necessary
* 558 x 270px (Wide)  - is used for the `Wide` tile as, in this case, resizing the 558 x 558px image just ends up looking weird
    
The two images used are 558 x 558px / 558 x 270px because that is the recommended image size in order for the tiles to look good on a wide range of devices.
    
For more information about  `/browserconfig.xml` and tile images: 
 * http://msdn.microsoft.com/en-us/library/ie/dn455106.aspx
 * http://www.buildmypinnedsite.com/ (if you want to view how different sized tiles look)

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For now I just wanted to see what you guys think about this additions. I'll refine this pull request as we go along (adding/updating the related docs, changes things based on suggestions etc.)