Add accessor functions for canvas and ctx within the CanvasRenderer class

Since the `barColor` function already has access to `this.renderer`, we were just one step away from being able to use a canvas gradient in our `barColor` definition. With this minor change (allowing outside access to ctx and canvas) we can use a gradient fill, too.

Check out an example here: http://www.codepen.io/stereoscott/pen/wiKHD

```
var element = document.querySelector('.chart');
new EasyPieChart(element, {
  barColor: function(percent) {
    var ctx = this.renderer.ctx();
    var canvas = this.renderer.canvas();
    var gradient = ctx.createLinearGradient(0,0,canvas.width,0);
        gradient.addColorStop(0, "#ffe57e");
        gradient.addColorStop(1, "#de5900");
    return gradient;
  },
  trackColor: '#cccccc',
  scaleColor: false,
  lineWidth: 40,
  lineCap: 'butt',
  size: 400,
  rotate: 180
});
```

It's not a 360 curved linear gradient, but still provides a nice visual effect.
cc: https://github.com/rendro/easy-pie-chart/issues/40