Add test for importing PGM image
Replicate provided 'example.c' in Ruby

The new 'example.rb' inputs a PGM image and applies thresholding to it.
Add characterization tests for the threshold test
Only test key thresholding values
REFACTOR threshold.rb -- extract function
Ignore temp directory
Add an Image class
WIP rename variables
WIP extract function
Remove unused constants
WIP Add width and height methods to Image
Import image from PGM file, get height and width
Use Image for thresholding
Use blocks for file input & output
Reformat tests to be independent of example c program
Test to_raw against from_raw