Render chart without Forms pop up.

RE: https://stackoverflow.com/questions/19649525/fsharpchart-saveas-saves-blank-image-if-called-before-chart-rendering-is-comp/44184418#44184418

Issue of being unable to save image without calling SaveChart() and pop up Forms window.

```
/// Render the chart in a new ChartControl without requiring a forms window
member ch.RenderChart () =
let frm = new Form(Visible = false, TopMost = true, Width = 700, Height = 500)
let ctl = new ChartControl(ch, Dock = DockStyle.Fill)
frm.Controls.Add(ctl)
```

suggested fix also slightly hacky, thoughts?