First stub of a length scale

It can be used whenever fixed segment lengths are drawn. Examples are given 
with geom_rug but it could also be used with a segment geom that would be 
defined in terms of angle and length rather than in terms of beginning and end 
coords.

The remaining issue is the legend, which currently just draws segments. It 
should draw segments of lengths identical to the main plot but since the two 
viewports (main plot and legend) do not have the same reference, it is hard.

Edit on 2014-02-27: rebased on head + latest version of roxygen