Add exclude argument to SpriteFactory config

[AlexanderRD]

Added an 'exclude' argument to the SpriteFactory class.

All image names within the target directory will be checked against the exclusion array, and images whose name match, or contain, elements from within the exclusion array will not be included in the sprite or stylesheet.

See tests for further details and for an example.

Use Cases:

1. User wishes to generate a sprite that contains only a sub-set of the images contained within the target folder. The user makes use of the :exclude argument to exclude certain unused images from the generated sprite. The sprite generated is smaller in size, hopefully resulting in faster load times. Unused images can be left in the target folder until they are needed, but won't bloat the sprite.

2. User wishes to create a sprite where the target folder matches the output folder. For instance the target folder is the Images/Icons folder, and the output target is Images/Icons/Icons.png. By using the :exclude => "Icons.png" argument future sf commands will not include the existing sprite in the newer sprite.

[jakesgordon]

Thanks for the great PR - nice and clear and includes tests! Thanks.