Sketches in basic source <-> target highlighting crosswalks.
Renames groups -> wrappedGroups.
Revert "Renames groups -> wrappedGroups."

This reverts commit 365a0ffba9cf74771ef77db6f1f4ffb95c5058de.
For now, collapses the (un)highlightSouce/Target methods in Avatar.
Implements grunt-concurrent, which makes it possible to run the watch and harp tasks in parallel.
Stub for AvatarGroup.
Renames _selectGroup -> _buildGroup, for clarity.
Starts to sketch in the selection logic for the page segments.
Flattens out the /lib file organization, sets the stage separate span and segment avatar classes.
Adds/implements a Segment class, to match the Span class.
Implements SegmentGroup#_buildGroup().
Segment positioning working.
Branching logic for first/last page rendering in SegmentGroup.
Page segment rending working.
Stubs in a factored-out Group class.
Makes SegmentGroup and SpanGroup inherit from Group.
Implements (un)highlight events on the segment avatars.
Stub for a Window class.
Adds a getScaleRatio method to Heatmap.
Minimally working window.