Rigging Goodies: Spline IK Constraint

At last, this commit introduces the Spline IK Constraint to Blender. Spline IK is a constraint that makes n bones follow the shape of a specified curve. 

Simply add a chain of bones, add a curve, add a Spline IK Constraint to the tip bone and set the number of bones in the chain to make it work. Or, try the following test file:
http://download.blender.org/ftp/incoming/250_splineik_spine01.blend

Screenshots of this in action (as proof):
http://download.blender.org/ftp/incoming/b250_splineik_001_before.png
http://download.blender.org/ftp/incoming/b250_splineik_001_after.png

I've implemented this in a similar way to how standard IK solvers are done. However, this code is currently not an IK plugin, since I imagine that it would be useful to be able to combine the 2 types of IK. This can be easily changed though :)

Finally, a few notes on what to expect still:
* Constraint blending currently doesn't affect this. Getting that to work correctly will take a bit more work still.
* Options for not affecting the root joint (to make it easier to attach the chain to a stump or whatever), and non-uniform scaling options have yet to be added. I've marked the places where they can be added though
* Control over the twisting of the chain still needs investigation. 

Have fun!

release/scripts/ui/properties_object_constraint.py

@@ -581,6 +581,13 @@ class ConstraintButtonsPanel(bpy.types.Panel):
         row = layout.row()
         row.itemL(text="To:")
         row.itemR(con, "track", expand=True)
+		
+    def SPLINE_IK(self, context, layout, con):
+        self.target_template(layout, con)
+        
+        row = layout.row()
+        row.itemR(con, "chain_length")
+        # TODO: add the various options this constraint has...
 
 
 class OBJECT_PT_constraints(ConstraintButtonsPanel):