Introduce d3.Layer
Make Layer API more D3-like

Wrap Layer constructor function to promote better parity with D3.js code
style.
Re-factor bar chart to use d3.Layer

Implement minimal changes to take advantage of the newly-introduced
d3.Layer framework. Forthcoming refactoring will improve readability and
efficiency.
Re-factor chord diagram to use d3.Layer

Implement minimal changes to take advantage of the newly-introduced
d3.Layer framework. Forthcoming refactoring will improve readability
and efficiency.
Instantiate a second bar chart for demonstration

This demonstrates the configurability enabled by the `d3.layer` API.