Fixed error in "The Quotient Rule"

Differentiation of a fraction is wrong if the numerator or denominator independent of the variable differentiated with respect to.
It looks like somebody forgot to divide by (denominator)^2 for the special cases where one of the derivatives is 0
Added test of The Quotient Rule
Removed unnecessary check in differentiate{:^}
Merge pull request #13 from ivarne/master

Error in The Quotient Rule