core: Add 'default_colorspace' global option to set the default ImageMagick colorspace

In some ImageMagick versions, the default "sRGB" color space results
in resized images that are too dark.

The new default_colorspace global config setting allows to override
the color space with another one, like "RGB".

This solution was inspired on the similar fix in Typo3:
http://forge.typo3.org/projects/typo3v4-core/repository/revisions/64f5f361a2468f7352ea505fec90e76733d75c34
(cherry picked from commit bcc12c158696ef4642431da3b23bbe7e3c619857)