Clarify lifecycle event selection process

Previously, the name "selection" described both a D3.js selection *and*
a function that returned a D3.js selection. This organization reflects
the distinction more clearly. (It also avoids the overhead of creating a
new function via `Function#bind`.)

Add comments to more explicitly explain the intent of the code.
Merge branch 'clarify-layer-selection' of github.com:jugglinmike/d3.chart into jugglinmike-clarify-layer-selection
Merge branch 'jugglinmike-clarify-layer-selection'