Optimize images within multiple directories

Closes the following issues:
* #23
* #108


You can now pass in any additional image directories. Any .jpg or .png within those directories get's optimized.

## Example

Simply edit project.properties, set a comma seperated list of directories, if there's only a single directory everything should run fine.

#### project.properties:
dir.images = img, css/images, css/img

Every .jpg/.png within img, css/images and css/img will now get optimized.


I've tested this thoroughly on Windows, I did make some modifications and removal of duplicated code within -imagesjpg and -imagespng, it would be great if somebody could test this on a *nix system.