grid and axes improvements

Some improvements to grids and axes. 
I think, for readability sake, this should be:
```
xgrid = Grid(plot=plot, dimension=0)
if xax:
    xaxis = LinearAxis(plot=plot)
    plot.below.append(xaxis)
    xgrid.ticker = xaxis.ticker
```
(unless this has different meaning, that I don't see).