Fix 'paddedimage' and 'setratiosize' image formats

- Fix VersionedImageTest unit test
- Support for $color argument in 'paddedimage'.
- Simplified 'setratiosize' case, since looking at
Image->SetRatioSize(), $width and $height are the only
arguments, and required - so the for retrieving those values
from $arguments seems unnecessary.

See https://silverstripe.atlassian.net/browse/CWPBUG-97
Merge pull request #28 from chillu/pulls/imageformat-test-fixes

Fix 'paddedimage' and 'setratiosize' image formats