add findByCoordinates API GET request.

In find by coordinates the user can get the closest cities to a given
lat / lng

[fixes #54]
Changed the find by coordinates to use a (query) request object.

also changed the lat and lng order in the mongoDB aggregation.

[fixed #54]
Fixed the findByCoordinates method to use the 'limit' query param.

this fixes a bug - the limit was set into req.params and not in
req.query Express setting.

[fixes #55]