Various usability fixes for scrolling/menus

+ Fix Z-index of TOC drop down
+ Don't do scrollTo stuff unless pushState is supported
+ Normalize browser behavior w/r/t updating hashes, storing position,
returning to position, etc. using history.replaceState and
history.pushState to store scroll positions
+ Add a listener to kill all animation if you wheel/touch while the
HTML is animating
+ replace window.scrollTo with less potentially-screwy window.scrollBy
to fix iOS non-scrolling issue

Related to issue #10