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`?
BTW per FD discussion we are going to have three functions, something along the lines of:
```
line = Line(x="x", y="z", line_color="red")
wedge = Wedge(x=0, y=0, radius=1, line_color="white")
plot.add_glyphs(line, wedge)

plot.add_layout(LinearAxis(), 'below')
plot.add_layout(LinearAxis(), 'left')

pan = PanTool(dimensions=["width", "height"])
wheel_zoom = WheelZoomTool(dimensions=["width", "height"])
plot.add_tools(pan, wheel_zoom)
```
tho `add_glyph` will need a little more thought