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

Target milestone: After 1.2 release.

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. This is necessary to provide a more robust means of determining whether or not an element is visible or not, which `height()`/`width()` need to know.