Made a pass over the Interactive user guide

[jbednar]

Interactive.ipynb looks great! Here I've made mostly minor cleanups and clarifications. The biggest addition is a Panel app at the very end. I think it's important to show that .interactive isn't a dead end, i.e. that it can be reconfigured and used in a larger app the same way as interact() can. Remaining issues:

• The text doesn't explain "accessor"; it probably should!
• I think there should be an example where multiple plots are shown in a layout. I think that would require referring to an inferred DiscreteSlider widget so that it can be passed into another .interactive call, but that seemed a bit hairy to explain.
• The Panel app I made isn't gorgeous; @philippjfr may see some simple way to make something more compelling.

[coveralls]

Coverage decreased (-0.2%) to 75.263% when pulling e8864b1 on interactive2 into a4fb538 on master.