Add Chai as a devDependency for better AssertionError messages

I've always found the failure messages in Chai to be superior to `node.assert`, especially when dealing with collections. Chai uses `diff` to produce much more readable output when `assert.deepEqual` fails, for one.

Since this change wasn't particularly related to the original in #128, I opened a separate PR for it, but the branch is still based on `al-the-x:patch-1`. Can rebase as needed.