Bump version number to 3.1.

Remove unused variables

Fixes mozilla#142

Bump version to 2.1.

Bump version number for 2.0, make this version 2.0.1.

Update MS-SSIM tuning

Change optimization parameters when tuning for MS-SSIM for better
performance

Fix mozilla#25

Scan pattern needs to be set again after changing colorspace

Optimize scans by default only if Mozilla defaults enabled