Orbitable base class Resolves #133

Please see issue #133 for a long, full list of the user-visible changes.

The reason so many files are affected is that the constructors had to
be changed slightly so everywhere a VesselTarget or BodyTarget are created
was modified a little.  Also a few places were changed to operate on the new
generic Orbitable object so they now operate on Bodies as well as Vessels.
I thought I told git to ignore the kOS.sln file because SharpDevelop keeps changing this line, but it seems to have been added in to the commit anyway.