Added function to determine Nyquist plot

Code based on the lti.bode() function.
n=10000 decided through trial and error. Might be somewhat high for
simple systems.
improved nyquist() function

added testing and changed return values as suggested by rgommers and
ewmoore
changed name to freqresp()

Changed the name as suggested on the Mailing List and adjusted the
docstrings
freqresp() now uses freqs()

Also changed order of return values to match that of freqs() to keep it
consistent
fixed error in docstring

forgot to adjust the example to the changed order of return values

Also i just saw, that bode has already been added to __init__.py so I
took that back out... Sorry
fixed testing for frequency range

Also adjusted docstring and examples to have consistent names with
freqs()
PEP-8 conformity

Thank you WarrenWeckesser for pointing those out.
PEP-8 Style change
removed docstring from tests

As asked by rgommers
Merge pull request #418 from Niklatz/master

New function: freqresp for lti-systems