ENH: add slider option in navigator argument for plotting

As discussed in https://github.com/hyperspy/hyperspy/pull/145 plot(navigator=) behavior was not working as expected. 

I changed the behavior for navigator=None, which now will not give any navigation image or slider. And I added navigator="slider", which will behave as None used to.

Does any functions use the "old" navigator=None functionality? If so, we need to change them so they use the new "slider" argument. 

I also need to change the docstring, and add some documentation in the user-guide.