Added tutorial about working with renderers and plots

[philippjfr]

Initial draft of a Tutorial about working with renderers and plots. Needs review. Here's a rendered version: https://anaconda.org/philippjfr/plots_and_renderers/notebook

[philippjfr]

I'd like to get this integrated soon, probably as a user guide since it's already very API focused. @jbednar correctly pointed out that it was a bit of mess and a bit repetitive (largely because it contains copied and pasted bits from 3 different versions of similar documents). Suggestions on how to make quick improvements to this very welcome. The main reason I'd like to see this merged is so I can reference it from the user guides about bokeh server deployment, to avoid having to introduce the Renderer concepts there.

[jlstevens]

I would like to make a pass through this notebook but I do understand you want it merge ASAP. Here is what I suggest:

1. Don't add this to Tutorials, add it to examples/userguide.
2. Write the user guide reference material in it.
3. When I make a pass through it, I'll have to make sure to update the user guide accordingly which means I'll wait till you have your user guide merged.

Maybe we can have a PR after your first user guide version where I do a pass through this notebook, you can review it and then we can update the user guide accordingly.

[jlstevens]

@philippjfr Is this material now in the user guide?

[philippjfr]

Yes, it is. Still needs polish but we can close this.