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`?
Are you suggesting making `plot.left` a setter property, just so I understand?