spatial: uniformize constructors and initializers

Several changes:

 * add Zero() initializers to Force, Inertia and ltI.

 * change the behaviour of ltI constructor, which was initializing the values to
   zero. Now it lets them uninitialized.

 * clean up ltI a bit: make it a class instead of a struct, remove tabs, simplify
   computations.

Note that Motion already had a Zero method and Transform had Identity.

This fixes issue #41.
spatial: fix style
robotbuilder: replace auto_ptr with good old ptr

because the auto_ptr causes warning C4251: need to have DLL interface
on visual studio.