Trac 28709

Implements 2 todos in customize-controls.js : 413
In wp.customize.Panel.toggleExpanded(), the sibling sections and panels must be collapsed before the panel is expanded.
This pull request uses hasExpandedElement() to check that the siblings are collapsed. If they are not, it sets a 250 millisecond timeout before executing expandPanel().

Amends #29.
@westonruter: Accounting for multiple expanding sections seems like a good idea. Your design sounds good.

I think api.section.expanded would have to be an object:
`{ expanded : false , duration : 0 }`
Then `api.Section.collapse( { duration : 0 } )` would set the expanded object, and `onToggleExpanded` would handle it.