reorganize modal views so they don't share the global modal element, but instead get pasted in when called with 'show'. much cleaner approach.
add addChild method to base view so we can add children without actually rendering them -- need this for modals.
forgot to return the view from render in sign-in view.
rather than rendering modals, just have modal render itself when shown.
need to re-delegate events when showing modal because bootstrap modal seems to unbind them.
make sure to empty any html in the modal before showing it.