Implemented two-fer exercise

[herwinw]

This replaces the old hello-world that is simplified in #69.

[herwinw]

The build fails with:

-> An implementation for 'two-fer' was found, but config.json does not reference this exercise.

If someone could review the code, then I'll figure out how to update the json file

[benreyn]

Ill take a look @herwinw. Thanks for the PR!

[benreyn]

This looks good codewise @herwinw. Once we get the exercise added to config.json I think this should be good to merge.

[herwinw]

Well, that was about 2 minutes of work after 30 minutes of searching (link). I've added the exercise to the bottom, it will probably need some reordering, but that should be covered in #45 since that problem is bigger than just this exercise.

[benreyn]

Thanks @herwinw!