Keep an internal reference to Highcharts

This allows module loaders to clear the global scope after Highcharts and its plugins have loaded without causing issues.  Without this patch, `window.Highcharts` *has* to exist, even after Highcharts has loaded, making it impossible to ensure that your modules are only acquiring their dependencies from the module system (rather than the global scope).

See [this JSFiddle](http://jsfiddle.net/Vt7b4/1/) for an example of the bug.