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

- 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.
feat: support removing children from fa-header-footer-layout

Merge branch 'patch-7' of github.com:bguiz/famous-angular into bguiz-patch-7
chore: update build
test: add unit test covering fa-header-footer-layout's child removal
chore: add todo