Buffers back on map

This puts stop buffers back on the map; it does so by refactoring the SystemGeometries class to be more extensible -- thanks to @echeipesh for the tip about abstract type members, that made everything easy.

This also deletes a bunch of commented-out tests from opentransit-test which I don't think are needed any longer.

This increases indicator calculation times somewhat because stop buffers are now being calculated twice (once in the indicator, once to create the geometry that gets saved to the database). I couldn't find an easy way around that, but it only adds maybe 15-20 seconds for Zhengzhou. If it starts to be a problem for bigger cities then it would be a good place to look for performance improvements.
![screenshot from 2014-11-04 09 37 23](https://cloud.githubusercontent.com/assets/447977/4901750/b33cda6c-6433-11e4-8027-65a6d97a5dae.png)