Fix scrollbar width calculation
Fix scrollbar width calculation

This PR does a few things:

- fix the code in `measureScrollbar()` which was broken (must have been some recent chrome version). I replaced it with code from the Google Closure Library (see https://stackoverflow.com/a/21046991/460084)

- added options.viewportClass to allow styling the viewport. This allows changing the scrollbar appearance (in chrome & IE) for example:

`
		.visibleScrollbar::-webkit-scrollbar {
			width: 14px;
			background-color: #F5F5F5;
			border-left: 1px dotted silver;
		}
		.visibleScrollbar::-webkit-scrollbar-thumb {
			background: #D3D3D3;
			border: 2px solid #F5F5F5;
			border-radius: 0px;
		}

		.invisibleScrollbar::-webkit-scrollbar {
			display: none;
		}
		.invisibleScrollbar::-webkit-scrollbar-button {
			display: none;
		}
		.invisibleScrollbar {
			-ms-overflow-style: none;
		}
`

and then use it in the grid:
`
options.viewportClass: isMobile ? 'invisibleScrollbar' : 'visibleScrollbar' ,
`

- some code in `init()` that relied on `measureScrollbar()` needed to be moved till after the viewport is created and `scrollbarDimensions` calculated
6pac/SlickGrid,yamass/SlickGrid,6pac/SlickGrid,yamass/SlickGrid