Code review for the mobility-service-interface

This patch adds the mobility-service-interface (RoutesMobilityModel)[1,2] module to ns-3.
The mobility-service-interface generates realistic mobility traces by querying the Google Maps APIs to request directions information between two real world locations.
The information obtained from these APIs is then parsed into ns3::Waypoints, that are then used with the ns3::WaypointMobilityModel, thus generating mobility based on real world routes.
More information on this module can be found on the wiki[2] and the module documentation.
This module features:
 * Simple mobility generation for a node
 * Automatic mobility generation for node containers
 * Generation of mobility traces using local XML response files

One test was developed, to test for the conversion of the WSG84 coordinates to Cartesian coordinates, the time placement of the ns3::Waypoints and the polyline conversion algorithm.

Three examples illustrate typical uses for this module
 * routes-mobility-example.cc
 * routes-mobility-automatic-example.cc
 * routes-mobility-offline-example.cc

A Google Maps API key must be created in order to use this module. Information on how to create a Google Maps API key is on the module's sphinx documentation.
The API key must be placed in the file src/mobility-service-interface/conf/api-key.txt

This module depends on three third party libraries, that need to be installed in order to use it:
 * GeographicLib
 * Xerces-C++
 * curlpp

[1] - http://www.cister.isep.ipp.pt/docs/routesmobilitymodel__easy_realistic_mobility_simulation_using_external_information_services/1060/attach.pdf
[2] - https://www.nsnam.org/wiki/RoutesMobilityModel
[3] - https://bitbucket.org/TiagoCerqueira/routesmobilitymodel/overview

[tiago.cerqueira, 2015-08-27 23:41:41]

Hi,
This patch adds the mobility-service-interface (RoutesMobilityModel), discussed
at WNS3 2015.
The module was developed under the SummerMentorshipProgram 2014, mentored by Tom
Henderson and Scott Carpenter, and this issue improves on the previous code
(issue #176430044), with bug fixes and several added features.

I also made some changes to doc/models/Makefile to include the images for the
sphinx documentation

I'm open to any questions you might have on this module!

Best regards,
Tiago Cerqueira