Terrain contact positions

Changes:
* Add the dev terrain class `RigidBodyFlatTerrainNoGeometry`
* Add a `terrain_only` option to `RBM/collisionDetect` (See help for that file)
* Exclude the world when computing terrain contact points (Fixes #146)
* Makes `collisionDetect` work with `TaylorVar` inputs (Fixes #142)
* Replaces `contactPositions` and `contactPositionsJdot` with `terrainContactPositions`
* Populates C++ contact points with the terrain contact points of the manipulator
* Improved documentation (See `RigidBodyGeometry/getTerrainContactPoints` for a description of terrain contact points)
* Misc. bug fixes

Passes all tests (modulo known issues) on my machine (Ubuntu 12.04, MATLAB 2013b)