Add a screenshot script for our examples.

This script is supposed to be executed via phantomjs. It will generate
screenshots of the html files in the directory specified by a commandline
option when these files are served through a webserver which can also be
specified. The screenshots will be in the current working directory.

Example usage:

    user@host:~/ol3/tools $ phantomjs example-screenshot.js \
                            http://localhost:8000/ol3/examples/ \
                            ../examples/

The above command will generate `*.png` files in `tools/` for all html files
that are served through `http://localhost:8000/ol3/examples/`.

So if there is a file `my-humpty-example.html` inside of the relative folder
`../examples/` which is being served through the webserver so that
`http://localhost:8000/ol3/examples/my-humpty-example.html` is a valid and
reachable URL, this script will generate a screenshot and store it as
`my-humpty-example.html.png`.

The query string `?mode=raw` will be appended to the generated URL.

Known limitations:

As phantomjs doesn't support WebGL (see e.g.
https://github.com/ariya/phantomjs/wiki/Supported-Web-Standards and
http://code.google.com/p/phantomjs/issues/detail?id=273) This won't render
OpenLayers maps rendered through the webglrenderer.

In parts based upon this gist: https://gist.github.com/crazy4groovy/3160121

Here is an example output of the script:

    Capturing 21 example screenshots.
    [>                    ]   0.0 %
    [->                   ]   4.8 %, anchored-elements.html
    [-->                  ]   9.5 %, animation.html
    [--->                 ]  14.3 %, bing-maps.html
    [---->                ]  19.0 %, brightness-contrast.html
    [----->               ]  23.8 %, canvas-tiles.html
    [------>              ]  28.6 %, epsg-4326.html
    [------->             ]  33.3 %, export-jpeg.html
    [-------->            ]  38.1 %, full-screen.html
    [--------->           ]  42.9 %, geolocation.html
    [---------->          ]  47.6 %, hue-saturation.html
    [----------->         ]  52.4 %, mapquest.html
    [------------>        ]  57.1 %, mouse-position.html
    [------------->       ]  61.9 %, rotation.html
    [-------------->      ]  66.7 %, scale-line.html
    [--------------->     ]  71.4 %, semi-transparent-layer.html
    [---------------->    ]  76.2 %, side-by-side.html
    [----------------->   ]  81.0 %, simple.html
    [------------------>  ]  85.7 %, stamen.html
    [-------------------> ]  90.5 %, tilejson.html
    [-------------------->]  95.2 %, wms-capabilities.html
    [---------------------] 100.0 %, wmts-from-capabilities.html
    21 screenshots captured.

And some example images:

![anchored-elements html](https://f.cloud.github.com/assets/227934/248893/89948372-8b1d-11e2-8c7e-7fab5b0afb5e.png)

![bing-maps html](https://f.cloud.github.com/assets/227934/248896/90e0c6d6-8b1d-11e2-976c-33484f5d2e0a.png)

![stamen html](https://f.cloud.github.com/assets/227934/248901/a25792a0-8b1d-11e2-8731-35552ca08651.png)

![rotation html](https://f.cloud.github.com/assets/227934/248898/94a84140-8b1d-11e2-93cc-ea81dcf3a7f3.png)