Revamp Plot script

Relying on the svg.charts library proved too cumbersome.

Now that we have a nice method to convert a date into a x coordinate,
we can simply generate a SVG path, which can be directly used in
our Timeline.
Update members data
Add debug export to Makefile

Add Makefile target for debugging purposes: when you have no
idea where your elements went, it is convenient to export the full
page in PNG in order to spot them.
Extend drawing area and add markers
Add board members following 2013 AGM
Update staff data
Update members plot
Update PNG preview