Calculate poles more smarter for state-space systems

As pointed out by @pmli in #27, the [current implementation](https://github.com/python-control/python-control/blob/0b26e062a6cd82da8a2420ebc3957e5267e6e116/control/statesp.py#L408) involves unnecessarily computing the characteristic polynomial, and then finding its roots.  This is inefficient and numerically unstable.  Much better to use `np.linalg.eigvals`.
