Expose WebGL context options via CesiumWidget and Viewer

Also cleaned up some Specs, particularly for Scene and added specs for context options to all three classes.

[Bug 745912 in Firefox](https://bugzilla.mozilla.org/show_bug.cgi?id=745912) is causing the new contextOptions specs to fail. (They pass fine in Chrome).  How do we handle situations like this?  Do we just let Firefox fail?

CC @shunter @pjcozzi