Add an ImageMagick driver

This library invokes `identify` and `convert` directly via Process.spawn,
requiring ImageMagick to be installed but requiring no additional gems.
Add padding and margin options to the cli interface
bump version number to 1.6.0
added a Gemfile (with development dependencies)
Merge branch 'imagemagick' of https://github.com/willglynn/sprite-factory into 1.6.0
Merge branch 'master' of https://github.com/miguelgonz/sprite-factory into 1.6.0
started 1.6.0 release notes
updated release notes