Added leaflet map and layers

This MR is kind of a large one even though it only adds the map to the app.

• Added the leaflet mapping framework
• Added two overlay layers to the map: Bing satellite imagery and the OBM all buildings layer
• Added class Application to handle everything map related
• Added a test suite for the application file
• Replaced the old hello-world test suite in the index file with real tests that test the app
• Switched style sheets from css to sassy csss (scss) files for better pluggability

\approve @marius @fd \fyi @ds @nicgar