Improve binary stl file detection.

Can not rely on information is the stl header to
determine file type.
Improvements to binary file detection

  * Comments to explain magic numbers 80, 4 and 50
  * rescue exceptions
  * open file in binary mode
Use literal 4 for the number of bytes read for face_count
Comments and Exceptions
Merge pull request #99 from jimfoltz/feat/improved-binary-file-detection

Improve binary stl file detection.