New image comparison algorithm

Key points:
- `gm compare` replaced with `looks-same` package. `crop`/`size` for
now still use Graphics Magick, moving off it is a separate task.
- `tolerance` setting is removed in favor of "strict mode": by default,
only noticable changes (according to ciede2000 metric) will be treated
as failure, to treat all changes that way user can enable
`strictComparison` option.
- in case if some element in focus during test can potentially have
blinking caret displayed, diff will try to ignore caret.
Update readme and changelog
Merge pull request #67 from bem/feature/new-diff

New comparison algorithm