Added us liquid measurements and tests

Needed us liquid measurements so I added them in.

I noticed there is a disparity between other libraries and the published NIST coefficiants

For instance:
http://physics.nist.gov/Pubs/SP811/appenB9.html
http://en.wikipedia.org/wiki/Quart#United_States_liquid_quart
https://github.com/gentooboontoo/js-quantities/blob/master/src/quantities.js#L113

All have varying levels of precision. 

I matched liquids up to the more precise values in js-quantities so that I could match our front-end. Perhaps this is not what you want.