`Set()` has a name clash with the ES6 `Set` object

[nigelmegitt]

The class/function corresponding to the TTML set element is called Set, but there is already an ES6 standard object called Set, so this is likely to cause confusion or errors in the future (especially if we adopt ES6 as per #214). Consider renaming it to something without a name clash.

It's possible that there are other name clashes too; I haven't checked.

[BasBastian]

#229 might be the PR that solves that issue. Same goes for "Image" class, that is part of any modern browser's API