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`?
I guess I would add this: the object interface is very highly composable. But it is not completely arbitrarily composable. To make it more usable and provide better diagnostics we need to make meaningless combinations of objects difficult or impossible. That's the intent of the proposed helper methods. 



> On Aug 21, 2014, at 18:17, Mateusz Paprocki <notifications@github.com> wrote:
> 
> In examples/glyphs/anscombe.py:
> 
> > @@ -53,27 +53,37 @@ def make_plot(title, xname, yname):
> >      plot = Plot(
> >          x_range=xdr, y_range=ydr, data_sources=[lines_source, circles_source],
> >          title=title, plot_width=400, plot_height=400, border_fill='white', background_fill='#e9e0db')
> > -    xaxis = LinearAxis(plot=plot, axis_line_color=None)
> > -    plot.below.append(xaxis)
> > -    yaxis = LinearAxis(plot=plot, axis_line_color=None)
> > -    plot.left.append(yaxis)
> > -    xgrid = Grid(plot=plot, dimension=0, ticker=xaxis.ticker)
> > -    ygrid = Grid(plot=plot, dimension=1, ticker=yaxis.ticker)
> > -    line_renderer = Glyph(
> > -        data_source = lines_source,
> > -        xdata_range = xdr,
> > -        ydata_range = ydr,
> > -        glyph = Line(x='x', y='y', line_color="#666699", line_width=2),
> > +
> > +    xaxis = LinearAxis(axis_line_color=None)
> > +    plot.add_obj(xaxis, 'below')
> I'm not sure what's the right solution right now, but what we are touching here is much more general issue. Similar thing is size vs. radius.
> 
> —
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