Protect against Requirejs circular dependencies

[brannondorsey]

We seem to be safe for now as there are no circular dependencies right now. That said, we will likely encounter this again as we develop the library so there is a good tool call Madge (which we currently include as a devDependency in package.json) which traverses RequireJS source code and shows circular dependencies.

Usage is:

path/to/madge -f amd --circular path/to/src

i.e.

madge -f amd --circular ./src

RequireJS documentation describes how to account for circular dependencies in its documentation: http://requirejs.org/docs/api.html#circular