Define internals using the AMD module pattern in order to improve the
readability of the codebase while preserving the ability to test source
files directly.

In enhancing the project's internals, the previous commit also
introduced support for loading d3.chart from CommonJS environments.
Extend the test infrastructure to verify that this new module system is
properly supported, and formalize the module's exported value in AMD and
CommonJS environments.

This module is just a wrapper around the built-in
`Object.hasOwnProperty` method, making it somewhat superfluous. Remove
it in order to enhance readability.