Rearranged past events; added future events column.

This PR contains multiple visible changes to the front page, so I'd prefer that someone else take a look before merging it. (I did test with webby autobuild, and it looks fine to me.)

![screen shot 2014-11-13 at 10 10 14 pm](https://cloud.githubusercontent.com/assets/2104453/5041208/e58cdbde-6b81-11e4-8df4-e7c01d858088.png)

Summary of changes:

* The "Past Events" column wasn't scaling well, and it was difficult to easily see which events were in which years. And the reverse-chronological was also a bit hard to scan quickly; I changed it to forward-chronological. I tried stacking the years with fewer events into columns together but we don't need the space (yet).

* There was no place to put future events other than the "upcoming" map. For future events that are scheduled already but aren't soon (Minneapolis is one, although I'll submit that in a different PR), that's a problem; we want to inform people it's coming in July 2015, but filling the "upcoming" map with seven months of conferences won't scale.

* There was a promo banner for the Belgium 2014 event which, as it's past, doesn't need to be there anymore.

* Several 2011 events were incorrectly labeled as having been held in 2012. I corrected their labels on the "Past" list.

* Three events were still present in "upcoming" and needed to be moved off.

* An upcoming event was erroneously placed in "Past", possibly because "Past" was in a file labeled "_menu.txt". I've renamed that to "_past.txt" and added a "_future.txt", and edited the layout file to account for this. (I also dropped the redundant-seeming "Events" word since having it both on Past and Future looked awkward.)