Doc fix, bug fix, add CR2 support.

These are changes that came up while working on [camlistore](http://camlistore.org/).  Hopefully you find them useful.  

CR2 is tiff-like, so we can just skip the header stripping and everything works.  Check it out with the Canon files (only CR2 work, not CRW) at http://www.rawsamples.ch/index_en.php