Use subject in specs
Remove dog-2.jpeg as it will be created automatically
Add failing specs
Replace file when format changes
Add image case into own group
Add failing specs for multi page pdf
Add file and filename_changed? method
Add handling of format change at multi page pdf

File variable can be "filename-*.ext". So later following image manipulations will affect all layers.
Add documentation to readme
Do not replace file if conversion to same format
Clean up files after each example
Fix for inconsistent behaviour of GraphicsMagick

It also handles the case when `:layer => "0,1,5"`
Refactor #replace_file
Refactor #manipulate!
Merge pull request #15 from tmaier/feature/format-support

:format support for manipulate! (mogrify)