Avoid using string comparison in loops

Fixes #26
Added 2 seprate methods for writing faces:
write_face_ascii and write_face_binary. The correct method
is selected based on the type of stl file (ascii or binary.)
Bug incrementing @face_count in multiple places.
Use write method instead of puts for file output.
Merge pull request #83 from jimfoltz/develop

Avoid using string comparison in loops