=BG= impl remove child for fa-header-footer-layout

[bguiz]

• Fixes this issue (comment): https://github.com/Famous/famous-angular/issues/179#issuecomment-54391510
• ... without causing this issue, in another PR that attempts to fix the same thing, but breaks for me: https://github.com/Famous/famous-angular/pull/182#issuecomment-54584354

NOTE: It seems more right to do isolate.rendernode.something.set(null) instead of isolate.rendernode.something.set({}), however, this breaks, as the set function in RenderNode currently does not handle a null parameter.

[zackbrown]

This works well; thank you for this! I also added a unit test for fa-header-footer-layout child removal in 2f29afff6f6 . If we run into any issues down the road with .setting to {}, we could set to a new RenderNode() instead.