Multiplying and dividing units

** I merged this with the other branch I issued a pull request for. **

When multiplying or dividing units, this takes into account the operator being used (* or /) and takes into account the exponent and factors of each unit.

Added test case:
- 1.square_meter / 1.centimeter should equal 100 but equals 0.01


** P.s. Thanks for creating this Gem, it's been hugely useful (and fun) to build off of. **