WIP - Turrets

I've just made [a forum post](http://pioneerspacesim.net/forum/viewtopic.php?f=4&t=120&p=2265#p2265) about this for discussion purposes and with some instructions.

# Turrets:
This adds what I'm calling `Visual Turrets`, the code necessary to define what turrets you want, and where to attach them to a ships mesh.

It does NOT add support for actually firing/targetting/animating them, that can come later - these are static features for now.

# Test assets:
To try this code out you will need [some test assets](http://quantum-thoughts.net/Pioneer/vindictive.zip) that I have mangled into the format necessary, simply place it into your mod folder then either buy a ship ingame or open Pioneer up with `-mv vindictive` on the commandline to see it in the modelviewer

# The Disabled Turret Code:
Yes ok there is some turret aiming code in here, it came from an older attempt by jaj22 to add better gun mount support and turrets. These were viewpoint turrets like in Frontier so there was no external animating mesh etc. 
I think it's mostly good solid maths and will, probably, be the basis of an auto-aiming turret aniamting system so I've gone to the effort of doing the basic initialisation but it is otherwise DISABLED.
In the interests of getting this damned thing committed and us actually moving forward I'm happy it being in there so long as nothing crashes!

Comments expected and welcome here or on the forum :)

Andy