Resolving issue with ISO8601::Months#factor calculation when the duration is negative.
Resolving issue with positive durations where the base month plus atom are evenly divisible by 12 but greater than 12.
Refactor for readability
Merge #10

Resolving issue with ISO8601::Months#factor calculation when the duration is
negative. By mschile
Bump to version 0.5.1