Moved to consistently use ImageMagick and fixed image scaling regression

This PR is a mixed bag of many things that impact one another. It could have come in a different series but the process of development (beginning with fixing #9) lead to this united changeset. In this PR:

- Thin test suite by moving to utility functions
- Move to use ImageMagick for all external commands (previously mixed with GraphicsMagick)
    - Fixes #4 
- Updated README with ImageMagick change
- Added regression tests for #9 and fixed issue
    - Previously, we were scaling images instead of cropping and adding transparent padding

/cc @mlmorg @Raynos