Add labels to axle force figure

Add x and y labels and legend for forces on cart axle of hw 8.4. Add eps
file for generation of latex file.
Add hyperref package, solution for hw 8.4

Add hyperref package to homework solution 8 tex file. Add solution for
problem 4, parts a through d. Solution for 4d is incomplete and does
not include calculation of constraint forces. Listings simulate_cart.m
and cart_equationsofmotion.m are missing necessary changes to problem 4.