Use btConvexHullShape for boxes.

The convex interface that we're using for our closest-distance computations
gives erroneous normals for box shapes. If we represent the box as the convex
hull of its vertices the normals are fine. Fixes #174.
Add test case for box-point contact normals
Update contactImplicitBrick test.

Changes:
  * Removes terrain-only flag.
  * Disable visualization by default
  * Check positions and velocities with different tolerances (as per Michael's
    email)

Now passes with tolerances 1e-2 for positions 1e-1 for velocities.
Slightly increase tol in contactImplicitBrick.
Use rel. tol. 1e-8 in ModelTest.cpp
Merge pull request #267 from avalenzu/fix-contact-normals

Fix contact normals