add first User Case Four test
StringDigitMapper, keys function returns keys of the map
add Corrector class
Corrector methods for ILL and ERR account numbers
Parser sends ILL and ERR numbers to Corrector
In progress, Corrector to solve for illegible characters
Corrector replaces illegible characters appropriately
Remaining User Case 4 Tests Written
Corrector#correct_illegible returns string with ILL if valid account number not found
Corrector finds alternatives account numbers for ERRs
Corrector complete, User Case 4 tests pass
Merge pull request #1 from toreyhickman/use_case_4

Use case 4