Update fraction.js
Fix zero value bug

Before whenever you had a fraction 0/1 or something, it said it was not a number (NaN), which broke various parts of my code and did not enable me to multiply by zero, even if it wasn't in the denominator. I believe this solves all possible issues, but who knows.
Update fraction.js
Makes negative fractions display properly

new Fraction(-3,3).toString() => -1
new Fraction(-3,4).toString() => -3/4
new Fraction(3,4).toString() => 3/4

Everything now displays properly!
All the previous code is wrong

Well, most if it. this actually displays everything properly. Although 'new Fraction(1,0).toString()' is still shown as '-1'
Fixes divide by zero toString() bug

Now, new Fraction(0,0).toString() -> NaN
Merge pull request #6 from zwhitchcox/master

Update fraction.js