Add new methods for getSupportedUnits() and getAliases() to quantities and units, respectively
check types for quantity values and unit names.  Also disallow collisions of unit names when registering a new unit
add support for autogenerating metric values
name all the native unit variables, for convenience
missed current in the last commit
add SI unit prefix generators to the base SI physical quantities
fixed typo 'mili' units
decided not to support Si prefixes on arcminutes
refactor unit tests for quantities.  Add testing for all supported units and aliases
improve test coverage on PhysicalQuantity
PHPCS standards
Merge pull request #29 from triplepoint/feature/metric-multipliers

Feature/metric multipliers