Refactor OrphanImporter#extract_orphans

+ Remove unnecessary return
+ alter control flow - don't check for duplicates if no spreadsheet 
+ remove unused variable assignment
Swap return responsibilities in OrphanImporter

Returning false seems like the concern of the exception logging method, not of the add import errors method.
OrphanImporter#import_orphan refactor

+ Use #inject 
+ Use existing add_import_errors method where possible
+ Remove unnecessary variable assignment
+ Improve the check_for_duplicates loop
OrphanImporter - prefer string interp
OrphanImporter - concat strings with <<, not +=

This can be up to 25% faster, as += creates intermediary strings.
Fix typo in OrphanImporter spec
OrphanImporter: decrease public API size
Merge pull request #232 from nateberkopec/refactor-importer

Refactor OrphanImporter