[builder] Extended significantly the Armature builder helper

In particular, it is now (rightfully) a subclass of actuator, and it supports
adding IK targets to drive specific bones.
Add support for IK in armatures

Kinematic chains can now be controlled via inverse kinematics
by placing/moving so-called IK targets that are tracked by your
armature.

A new service (Armature.place_IK_target) allows to instanteanously
place and orient an IK target, while the asynchronous service
Armature.move_IK_target allows to move at a given speed the target.

The list of IK targets available on a given armature can be obtain
with Armature.list_IK_targets.

To easily create the IK targets, the morse.builder.Armature offers the
create_IK_targets([joints]) that can be called in Builder scripts.

Known bug:
- setting a rotation 'over time' (with move_IK_target) is currently
  diabled because I could not figure out a good way to interpolate the
  angles over time. Help would be welcome!

While here:
- removed the IK constraint in KUKA LWR Blend file
- added (from Python) an IK target to the KUKA LWR. Cf morse.builder.actuators.KukaLWR
  to see how this is done.
- updated testing/base/armature_testing
[makehuman] Initial support to load MakeHuman human models

To add such a human model, use these lines in your Builder script:

human = MakeHuman(path/to/your/mhx/model.mhx)
[...then it behaves like a regular robot. Armature control is already setup.]
[makehuman] Workaround for MakeHuman model rendering issue
[makehuman] Add IK targets to MakeHuman models for hands and feet

Currently introduces an infortunate 90deg rotation of the model...
[makehuman] Fix positioning of IK targets
[makehuman] Improve error handling when import MakeHuman models

- Properly check that the MakeHuman importer is enabled in Blender
- Check that the user set a skeleton in the model
[makehuman] Import only the lowpoly proxy mesh
[makehuman] Auto-activate Blender's MakeHuman importer if needed
[makehuman] Turned MakeHuman into real Robot-like

This is made possible by commit c25b05e
[makehuman] Fix renaming of IK targets