Tooltips

I implemented some basic tooltips, which show (if defined) the label and value onmouseover.
Every tooltip can be registered by a configurable area:
1. a simple rectangle: (x,y,width,height)
2. a circle: (x,y,radius)
3. any shape: (array of {x,y}  objects)

Tooltip Defaults can be overwritten by adding a 2nd parameter to the "new Chart()" function, i.e.

    var myChart = new Chart(context, { background: 'black', fontColor: 'white' });

The following Tooltip options are available:

    tooltipDefaults = {
        background: 'rgba(0,0,0,0.6)',
        fontFamily : "'Arial'",
        fontStyle : "normal",
        fontColor: 'white',
        fontSize: '12px',
        padding: {
            top: 10,
            right: 10,
            bottom: 10,
            left: 10
        },
        offset: {
            left: 0,
            top: 0
        },
        border: {
            width: 0,
            color: '#000'
        }
    }

The tooltip moves relative to the current mouse cursor position and shows only, if hovering a valid chart point or area.
Tooltips can be disabled via specific chart option "showTooltips" i.g.

    myChart.Pie(pieChartData, { showTooltips: false });

They can be styled individually and do right now a basic job:
Showing the hovered label and value.

![Screenshot from 2013-03-23 16:25:00](https://f.cloud.github.com/assets/1336590/294046/e6d3014a-93cd-11e2-9901-2dcf290889b5.png)