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`?
Well if you have a general solution I'd love to proceed but barring that we need to make some incremental usability improvements nowish because right now the experience is that it's basically impossible to use the glyphs interface correctly and even harder to get information about what's wrong when you don't. 



> 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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