Smallest positive real root of a quartic equation

This function returns the smallest positive real root of a fourth order polynomial (quartic) using Descartes' solution. For solving the the resolvent cubic I used a method from "Numerical Recipes in C: The art of scientific computing", page 184-185.
After some testing it on my own computer is seems that this function is between 5.5 to 6 times faster than using the MATLAB function roots, which also returns its complex roots.