Update tests and fix GetImages bug

Add additional directories and files for testing the GetImages class,
including files with the same names in different directories. Fix a bug
in GetImages that missed files with the same name in different
directories. GetImages now prepends the Job ID to each file name. Added
a method to remove the Job ID to clean up after tests.
Clean up logic for adding prefix to file name
Add more filename variations for testing
Rename only when needed and update feature spec.
Add Job ID to files randomly for testing
Merge pull request #31 from nulib/get_images_fix

Get images fix