Modify Pure Column API.

add getUnitNumerator and getUnitDenominator
Refactor get greatest common divisor logic.

Column::getGCDBy... to Utility::getGCD
Refactor ColsPlugin for sub-classes
Fix Pure-Utility
Add tests for Pure UnitsPlugin
Add Pure UnitsPlugin
Merge pull request #22 from toiee/add-pure-units

Add pure units