feat(jqLite): Add width/height methods to jqLite

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.