Elevation feature

Compared to the first pull request of Radu this version:
- has no ifdefs, appart one to select between _Point2D and _Point3D;
- documentation in Util/EstimateElevation.h;
- cucumber tests, notably in elevation.feature;
- code reuse, with new parameters, sometime optionals.

I still store the elevation as an int in _Point3D.
Due to struct alignment constraints, using short, in spite of int would still use the same amount of RAM.
Of course we might pack the structure, but is the small memory gain worth the probable misalignment and complexity slow downs?
Moreover,  @radupopescu showed the memory datastore only increases by 3%: https://github.com/DennisOSRM/Project-OSRM/pull/966#issuecomment-39090705


Regarding the pertinence of directly serving geometries with elevations,
@systemed and @eleu have commented at: https://github.com/DennisOSRM/Project-OSRM/pull/966#issuecomment-39197192


Regarding the cucumber tests, it may be nice to integrate a Ruby implementation of
the polyline. I have modified the one from @joshuaclayton to handle elevation:
https://github.com/gberaudo/polylines/tree/elevation
I guess we must copy the few files (BSD-style) in OSRM.
Could you advice if you want this integration and how to do it?