Get rid of old location model
On bikeway_segment#show, display KML on Google Maps.

Note it doesn't work on localhost since the Google servers have to actually fetch the KML. But if you go to `/bikeway_segments/2.kml` and load it in Earth you can see it works.

Tricky:

* Added `BikewaySegment#kml` method, which uses PostGIS to build a KML fragment (using st_transform). Even added an RSpec for it too (it's pretty brittle!)
* Used HTML5 data attributes to communicate the KML URL from Rails to JavaScript
* Used kml.erb to template out the KML file, yielding to the fragment from PostGIS in the middle