Added an accessor method for the margin parameter.

I use this one a lot in my code, so it made sense to me to add it to the list of properties that could be globally defined.
Added a sanitize names option to deal with source files that are named weirdly.

Sometimes the names of your source files are not really valid CSS selectors so if you treat then as such you're gonna have a bad time. Adding the :sanitizenames option makes it so the names are pre-processed to conform to CSS naming standards. NOTE: This is fundamentally incompatible with the colon and space escaping methods that were previously applied to the incoming file names.
Changed the assert_reference_image test harness to check for delta instead of equality.

It seems that some newer versions of imagemagick (or just it running on different OSs) changes the converted images very subtly. This change in the test harness makes it so that the images no longer have to be exactly equal, but need to be within a very close margin (currently 1e-7 or less percent difference) to pass the "equality" test.

Just for reference, in my case on a machine running OSX 10.9.2, MRI 2.1.1 & ImageMagick 6.8.7-7 Q16 the test_generate_sprite_from_other_formats test was within 7.8e-08 of exact. This was the only test that failed.
Added a test case for oddly named source files.

Copied a few of the same source files as some of the other tests but renamed them to have names that are not valid when used as CSS selectors.