Adds OS cursor support

Intercepts the `WM_SETCURSOR` message on the `HtmlLabel` and `HtmlPanel`
controls in order to override how the "hand" cursor is displayed and use
the OS version rather than the .NET one when hovering over a hyperlink.
Reinstated DebuggerStepThrough
Merge pull request #12 from cyotek/OSCursor

Adds OS cursor support