New function: nyquist for lti-systems

First time participating to a open source project. I hope I didn't do too many mistakes.

In the scipy.signal module I missed the ability to generate a Nyquist plot, so I made a quick implementation and figured it might be useful to others.

Code based on the lti.bode() function.
n=10000 decided through trial and error. Might be somewhat high for simple systems.

Like I said, it's the first time I participate in a open source project and I don't really know if I made any mistakes. I hope the code is useful even if it doesn't make it into scipy. At least I tried.

As attachment the plot that is given out by the example code in the comment:
![figure_1](https://f.cloud.github.com/assets/3473679/128012/1f3b7818-6fad-11e2-9540-559452c0f41f.png)