Add inline documentation and unit testing for `onMount`

[dpikt]

Old docs can be found in legacy-docs.md. Remember to delete the relevant section once it's been added to the auto-generated docs.

[davemcorwin]

This is a Higher Order Component (HOC) that provides the ability to do something during the onComponentDidMount lifecycle event of a wrapped component. We do this to keep as much of our application code as stateless, functional components as possible.

See the tests for onLoad for examples.

[wade00]

Word! I'll check out onLoad and get on this.

[dpikt]

Resolved in #22