Make Sandcastle demos prettier

I didn't like the jarring unstyled look of Sandcastle demo buttons and drop downs.
This is my attempt to make them look more like the rest of Cesium.
CSS tweaks.
Merge remote-tracking branch 'origin/master' into sandcastle-buttons

Conflicts:
	Apps/Sandcastle/templates/bucket.css
CSS tweak
resnap pictures
Merge pull request #1985 from AnalyticalGraphicsInc/sandcastle-buttons

Make Sandcastle demos prettier
Fix reference to leap seconds in documentation.
Remove animation controls from Custom DataSource.
The default HTML button type is 'submit' not 'button'.
Merge pull request #2001 from AnalyticalGraphicsInc/tweak-custom-demo

Tweak Custom DataSource demo