Fraction: Add skeleton/docs for parseFraction

I'm taking a documentation-first approach here so that I have my format
clearly defined before I start implementing the parser itself.
Otherwise, I run the risk of feature-creep.
Fraction.parseFraction docs: Clarify ignorance of leading zeroes
Fraction.parseFraction docs: Allow a single space to be a unit-symbol
Add a note-to-self about regex for a fraction
Update Fraction regex to recognize leading zeroes in denominator