Updated specs to validate sizing behaviour when maximum width/height is given (the default in imagemagick)
Specifically, if a size of 600x400 is requested the result should have aspect ratio maintained and fit within 600x400, not bigger.  Added specs to verify this contract and modified one incorrect spec.  Three specs now failing.
Fix geometry resize logic to always contain result within requested dimensions as stated by contract
See http://www.imagemagick.org/script/command-line-processing.php#geometry
Merge pull request #2 from robharper/fix_sizing_to_always_contain

Fix geometry sizing such that result is fit within specified maximum dimensions