Barcode class with implicit conversion to string
Moved barcode validation to Barcode class
Moved barcode formatting to Barcode class
Made validation private

Invalid barcodes should not exist anyway
Validation against a checkdigit

A little awkward at the moment, though, but working.
More accurate tests since validation became harder
Extracted validation logic
Removed redundant to_str
Calibrated coupling between barcode and validator
Reduced method complexity according to Flog
Some abstractions
Fix requirement
Tests for barcode validations
Merge pull request #3 from artemshitov/barcode-class

Barcode class, closes #2