each-* binder can now be re-bound

This patch fixes an issue causing the each-* binder to stop functioning
when the same DOM element is bound twice. Closes #261.

Changes to the each-* binder:

  - #unbind() now restores the source element that gets replaced by
    the comment marker in #bind()
  - #unbind() now tears down the child views spawned in #routine() by
    both unbinding them as well as removing them from the DOM