Blender Git Loki

This adds support for the new-ish options:
QUATERNION
B_BONE_REST
INVERT_VERTGROUP
MULTIMODIFIER

to the bpy armature modifier API.

It also fixes a significant problem - In the docs, it refers to
the 'VERTGROUP' field as being a string value, shared
by lattice, armature, etc, referring to the 'VGroup' field in the
armature modifier that defines a vertex group 'mask'
which the armature's effect is restricted to.

However, previously, for some very mistaken reason, in processing
the getters/setters for the armature modifier, the
VERTGROUP field was pointing to the quite different 'Vert. Groups'
toggle in the armature modifier, that enables or
disables using vertex groups for deformation (as opposed to
envelopes).

I've fixed this, so VERTGROUP points to the VGroup string, as is
already defined in the docs and consistent with other
modifiers like Lattice. A new field: 'VGROUPS' has been added,
which is the analog to the 'Vert Groups' toggle.