History refactor

* makes history into a singleton
* adds a summary of visited sections and (primary) keywords
* removes repeats from history, and instead:
    * moves repeated page to front of list  
    * maintains most-recent-first order
    * adds count of # of visits to each page
    * updates it's timestamp
* removes sequence.js

@phamann : I'm now adding __everything__ to history except network fronts and liveblogs..
* will adding section/tag fronts mess up your use of history?
* can I actually add liveblogs, now that deduping happens?

@NathanielBennett : I can't find any use of `recentVisits` or `numberOfSessionsSince` functions. Do you need them any more?
I could just do `window.history`, if I needed it.