Add `/browserconfig.xml` and tile images

By default, starting with IE11 on Windows 8.1, IE will request the
`/browserconfig.xml` file when a user clicks/touches the `pin site`
button (same as Safari for iOS does with the `apple-touch-icons` when
the user touches the `share` button).

Adding the `/browserconfig.xml` file and the tile images:

* reduces the number of 404s, especially as IE11 on Windows 8.1 gets
  more market share
* provides a starting point for dealing with Windows tiles
* reminds users about the tile images, so that their website doesn't
  end up having as a tile the default IE11 logo, or (if the favicon
  is 32×32px or bigger and not `.ico`) the website's favicon
* unlike using `<meta>` tags, offers a way that doesn't increase the
  page size

Only two images are provided since:

* `tile.png` (558×558px) - can be used for the `Small`, `Medium`,
  and `Large` tiles, being automatically resized where necessary
* `tile-wide.png` (558×270px) - can be used for the `Wide` tile
  as, in this case, resizing the 558×558px image would just end up
  looking weird

The two images are 558×558px and 558×270px because those are the
recommended image sizes in order for the tiles to look good on a wide
range of devices.

For more in depth information, please refer to the MSDN documentation:

* http://msdn.microsoft.com/en-us/library/ie/dn455106.aspx
* http://msdn.microsoft.com/en-us/library/ie/dn255024.aspx

Ref: h5bp/html5-boilerplate#1481.