Issue Description:

All other quantities, such as UnitDuration, UnitInformationStorage, UnitPower, etc. are key value coding-compliant with all the documented units of measure for the respective quantity.

For instance, UnitDuration is key value coding-compliant with the following keys:
hours, minutes, seconds, milliseconds, microseconds, nanoseconds, picoseconds

And UnitInformationStorage is key value coding-compliant with these keys:

bits, nibbles, bytes, kibibits, kibibytes, mebibits, mebibytes, gibibits, gibibytes, tebibits, tebibytes, pebibits, pebibytes, exbibits, exbibytes, zebibits, zebibytes, yobibits, yobibytes, kilobits, kilobytes, megabits, megabytes, gigabits, gigabytes, terabits, terabytes, petabits, petabytes, exabits, exabytes, zettabits, zettabytes, yottabits, yottabytes

However, it seems that UnitMass is mistakenly not key value coding-compliant with its key 'pounds', although it does comply with the rest of its keys:

kilograms, grams, decigrams, centigrams, milligrams, micrograms, nanograms, picograms, ounces, stones, metricTons, shortTons, carats, ouncesTroy, slugs