Add "mejs-shim" class to all shims to prevent improper resizing

We have a use case in which foreign iframes (overlay ads) are inserted into MediaElementPlayer's DOM structure. They need to remain a constant size, but there is some code that resizes all `<object>`s, `<iframe>`s, and `<embed>`s to 100% at times.

I've changed this code so every shim is created with a `mejs-shim` class, and they are selected as such, instead of selecting specific kinds of elements.