More additions for beta.

* Add sharrow description and image.
* Change bike lane and wayfinding image. (I took these photos, so they are released under whatever license works best with Streetmix. We'll have to be clear about this somewhere)
* Edit default street layout - removed the bench and put in a wayfinding sign, which has about-text
* Lame stab at help/about-beta text. We should go over this on Friday.
* CSS changes to get the +/- width buttons to line up horizontally with variant buttons in the segment popup.