Subassembly generation WIP
Select a model object.
Subassembly generation WIP.

Have defined new types of subassembly -- sheet, unit, void, infill.
Void type adds an empty Object3D. Unit type will require different
layout algorithms and specification of which algorithm to use in the
layer definition.
Center unitized layer on origin. Display layer bounding box.
Clip assembly to bounding volume WIP
Execute boolean operation on a layer to produce individual elements.
Added option to display assembly bounding box, layer bounding boxes.

Updated grunt tasks. Started producing README documentation.
Release v0.1.1
Frame layer WIP
Generate frame layer. Start adding support for shadows.
Merge branch 'develop'
Release v0.1.2