Refactor images in tests
Complete RDoc documentation
Rewrite README
Force (again) calling #format directly on image
Raise NoMethodErrors on MiniMagick::Image
Improve tests
Backport MiniMagick::Image#run_command

For compatibility with CarrierWave, it's simpler to just have this method
than managing breaking of CarrierWave.

Fixes #251.
Add MiniMagick::Image#layers

Concerns #144.
Refactor MiniMagick::Tool::OptionMethods
Encourage using ImageMagick's tools directly
Refactor MiniMagick::Image a bit
Remove PDFs and document MiniMagick::Image#layers
Make CarrierWave tests pass