allow clearing of a single renderlet or all renderlets

See #776 for rationale

This adds two new functions on chart:

```javascript
var renderletFn = function (chart) { /* do something with chart */ };
// add a renderlet
chart.renderlet(renderletFn);
// clear it
chart.clearRenderlet(renderletFn);
// clear all renderlets
chart.clearRenderlets();
```