[girv-client/frontend] Renderer now properly supports D3's
enter/update/exit paradigm.

Added grouping in order to prevent shuffling of different element types when D3
reorders.
[girv-client/layout] Added the ability to bundle a payload with a Layouter task.
[girv-client/core] Added IdMapping and GraphElementMapping classes. These
provide mappings between corresponding elements from two graphs.
[girv-frontend] Renderer now supports selective transitions based on a mapping
between the currently displayed graph and the graph associated with the render
request.

Viewer now uses an identity mapping for animating the transition when block
grouping is toggled.