Widget cleanup

1. The first parameter to each widget is now uniformly named and documented.  It can either be an element or string.
2. Each widget exposes a public `container` property, which is the element that was passed into the constructor.
3. `cesium-darker` is now the default theme, deleted `darker.css` files and replaced them with `lighter.css`  For each of use, there is now a `lighter.css` in the Widgets directory which just includes all other `lighter.css` files.

I plan on continuing widget cleanup, but decided it was better to make small, incremental pull requests rather than shoving everything together.
Merge branch 'master' into widgetCleanup
Changes after review.
Merge branch 'master' into widgetCleanup
Update CHANGES
Comment tweak.
Doc tweak
Maximum bike-shedding.
Change TimelineDemo to reflect proper theme names.
Merge pull request #580 from AnalyticalGraphicsInc/widgetCleanup

Widget cleanup