Fix pidplots for cases where plot type is not :controller

[non-Jedi]

Fix #226

Logic checking for :nichols was removed as nothing was being done with nichols_ in current code.

[coveralls]

Coverage decreased (-5.009%) to 53.004% when pulling c3b25ef on non-Jedi:patch1 into 9f3bd5d on JuliaControl:master.

[mfalt]

I think this looks good, can we pull this @baggepinnen ?

[baggepinnen]

I made some further changes so that the example in the docs works as intended. It produces all plots well on my local machine.