Add command line options
Read art from file
Use options in thread block
Save rendered image to output file
Only retrace if Lambertian factor >= 0
Constrain ray direction to an aspect ratio
Clamp pixel channel colors to 255
Cache common vectors
Update object offsets
Update rubyrays readme

rubyrays no longer writes to stdout by default.
Fix typo in megapixels description
Update readme to reflect the new output filename
Merge pull request #22 from zaeleus/rubyrays

rubyrays: Add option parser + other relevant changes