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.
Added "Model Units" as default export unit option.
Use write method instead of puts for file output.
Merge pull request #83 from jimfoltz/develop

Avoid using string comparison in loops
Removed commented code.
Merge pull request #84 from jimfoltz/feat-model-units

Added "Model Units" as default export unit option.
Fixed file reading bug where the UTF-8 flag wasn't being set.
Merge commit 'e4ab0a3977aeee973ba74c34b0c6e8145a8469c8'