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`?
If that's the case, and it returned some proxy for the list, so that you could do:
```
plot.left += some_axis
```
which causes the back reference to be set, and then appends to to a layout list (`plot._left`, I guess?) that might be ok.