Support fractional quantities.

This is to address Issue #17. The problem was actually that I typed in "1/4" as a quantity and the app crashed without saving the rest of my ingredients.  

Instead of converting directly to float, this change first parses the data as a Fraction, which is smart enough to deal with ints, floats, or numerator / denominator strings.  This doesn't work too gracefully with the up/down buttons, but at least it doesn't crash.

I also alphabetized includes 'cause I'm OCD.