Correctly remove attributes when deleting props

The most obvious manifestation of this bug is visible here:
http://jsfiddle.net/spicyj/zzGas/. In short, when props are removed from a
component, the underlying HTML element doesn't have the attribute
removed.

This change should fix it, but unfortunately it (presumably) makes
_updateDOMProperties a bit slower.
Fix relative require copy pasta
Remove unused require
internalPropNames isn't necessary, so remove it

Perhaps we'll bring it back as a future perf optimization if that
appears useful.
Fix nit and comment
Merge pull request #84 from spicyj/remove-props

Correctly remove attributes when deleting props