Use international standard symbol for gradian angle unit

Since ISO 31-1, superseded by [ISO 80000-3:2006](http://en.wikipedia.org/wiki/ISO_80000-3), the international standard symbol for grandian angles is "gon" ([Gradian - history](http://en.wikipedia.org/wiki/Gradian#History)). 

Also confirmed by The International System of Units (Si brochure), 8th edition, 2006, BIPM. ([PDF](http://www.bipm.org/utils/common/pdf/si_brochure_8.pdf) - p. 35, 124)

This pull request aims to update js-quantities for those who dare to use this forgotten french unit :smiley: