feat(jqLite): implement width() and height()

These are fairly commonly used DOM manipulation methods, which are sorely missing from jqLite.

The implementations provided here are much simpler than jQuery's implementations, relying only
on offsetWidth and offsetHeight, and performing no special operation for document elements or
other special cases.

However, they do provide some basic essentials, and this is a good thing.

Also implemented in this patch: jqLite.css() will now leverage `getComputedStyle` if it is available,
before relying on the inline style.