Lyapunov

Moved Riccati and Lyapunov methods to a matrix_eqn.py file.  Wrote some tests for lyapunov and corrected the calls made in quantecon to direct them to matrix_eqn.  Currently both are named `solve_discrete_()`, but I think using names like `dlyap` and `dare` would also be acceptable, I just wanted them to be consistently abbreviations or consistently explicit.