Broke out all dimensions into a separate file.

Now calculating the axel extension from servo measurements.
Added rear wheel, still have some tidying up to do on the constants.
Created rear wheel file, not happy with the shape at the moment, plan to change this substantially.
Made a working model for a larger wrist band

As said,  working scad model now available.  Use the settings in the
dimensions.scad file to set the dimensions for your bot.  Documentation
to follow.
Merge pull request #3 from Justblair/master

Updated further