Add a PdfExtractor class to replace JODConverter.
Move methods from class methods to instance methods.
Rename PdfExtractor#extract_pdfs to #extract
Add an office_path and raise OfficeNotFound if an office lib can't be found.
Directly shell to libreoffice if available, otherwise fallback to JODConverter.
Fix path to test_helper for ruby1.9
Fix method that wasn't moved from class to instance.
Fix typo screwing up path finding.