Add --output-dir option to output generated css files to a different directory

Hi,

When using compass as part of a build system, eg. in grunt or yeoman, it is often preferable to generate files in a temporary place, but yet generate them as if they had been created in the source. This keeps the source directory clean. The option --output-dir gives compass the ability to create its css and reference any assets as if the css was being created in --css-dir, but instead of placing the file there, place it in output-dir.