Add STL reader for Simbody's PolygonalMesh class

Fixes #57.

- Can now read ascii and binary STL mesh files (can force ascii with .stla suffix). Catches and reports most file format problems.
- Supports some STL extensions for Gazebo compatibility (comment lines, "color" keyword ignored).
- Added a new adhoc test LookAtPolygonalMesh that reads mesh files and tries to make contact geometry out of them (very fussy). If it fails it will try to explain why.
- Added PolygonalMesh::loadFile() method that examines the file extension to determine what format to try.
- Added PolygonalMesh::loadObjFile() signature that takes a path name (previousl required an open stream).