deal with prefixes when multiplying or dividing

[christianp]

Aims to fix gentooboontoo/js-quantities#94.

The method to multiply q1 by q2 goes as follows:

• if q1 and q2 are compatible (they have exactly the same signature), convert q2 to the same units as q1
• collect together the numerators and denominators of q1 and q2, and 'clean'
• multiply q1 and q2's scalars together, and return a unit that big with the cleaned numerator and denominator

The cleaning code doesn't gather together units of the same kind, but with different prefixes. I suspect this is deliberate, but it leads to inconsistencies like Qty('1 m').div('1 km') = 0.001, but Qty('1 m').mul('1 1/km') = 1 m/km.

This commit changes cleanTerms to keep track of the prefix used with each unit, and accumulate a scaling factor when the same unit is used with a different prefix. The function returns numerator, denominator, and the scaling factor.

Following the logic that km*m returns a quantity in km, and m*km returns a quantity in m, the prefix associated with the first occurrence of each unit encountered is used.

[gentooboontoo]

Thanks for your PR. I have published a new release after merging it.