Some usability cleanup

* [x] option for "simple" ids
* [x] remove necessitty to set `plot` on renderers
* [ ] remove necessity to set `plot` on tools
* [ ] cleanup, document and, test `bokeh.document`
* [ ] cleanup, document and, test `bokeh.plot_object`
* [ ] cleanup, document and, test `bokeh.session`
* [ ] remove `PlotContext`?
like what? setting the back reference to `plot` that is something that can and should be automated, even at the low level, that is what this is about. If you have a concrete proposal for specifying the layout lists I am all ears without that, this is clearly an improvement because it reduces the possibility of using things completely wrongly.