Implemented an image cache for test expectations.

[henryso]

Fixes #252.

There is a small, but significant, improvement on subsequent runs. Expectation images will be created in var/image-cache under the test directory.

Please review and merge if satisfactory.

[henryso]

FYI, on my computer, caching the converted images at 100 dpi saves about 8% time on subsequent runs.

[rpspringuel]

I use 150dpi by default and am getting something closer to 10% savings. I might have hoped for more, but I'll take what I can get.