add a hasClass() method

Two goals here:

First, the removeClass() wasn't adding spaces before and after element.className and the class. This could create weird false positives as we get into compound class names, like vjs-fluid, and vjs-fluid-4-3. This aligns the check across both methods.

Second, I think hasClass is going to be essential for testing, going forward.
Though, really, we should just return the comparison:
```js
return (' ' + element.className + ' ').indexOf(' ' + classToCheck + ' ') === -1;
```