Query the nearest stops

- Added support to the MongoDB adapter to find documents by proximity to a
  location (DB:Mongo#near)
- Added shortcut to obtain the stops that are close to a given location
  (Stop.near)

Issue #9.
Show the nearest stops in the application

Issue #10.
Fix for the stops consolidation algorithm

For some stops, the same bus line appeared more than once. I didn't check
why this was happening.

Issue #8.
Stop droping collections

In order to keep their indexes, remove all their documents instead.
WAT