added code to calc shannon entropy

this calculates the shannon entropy for the image, and adds it to the json output like this

<pre>{
    "average": {
        "closest": "#a0522d", 
        "colour": "#866340"
    }, 
    "palette": [
        {
            "closest": "#cd853f", 
            "colour": "#a6845f"
        }, 
        {
            "closest": "#deb887", 
            "colour": "#deb587"
        }, 
        {
            "closest": "#556b2f", 
            "colour": "#775c43"
        }, 
        {
            "closest": "#000000", 
            "colour": "#3c1a0b"
        }, 
        {
            "closest": "#800000", 
            "colour": "#672309"
        }
    ], 
    "reference-closest": "css3", 
    "shannon": 9.214914488876211, 
    "stat": "ok"
}</pre>