MG 2.0, re-render graphics on data change

Hi @jdjkelly,

I haven't spent time on the testing side of this PR, but I've been using your addon and had to improve a couple of bits:

- [x] Updated metrics-graphics to the 2.0.0 API
- [x] `{{data-graphic}}` component now rerenders when the `data` property changes, i.e., when a route reloads a new dataset from the server-side.

I'll keep adding tests in the upcoming weeks, and adding some bits of documentation to the `README.md` that I've found/learned when dealing with Ember addons.

Cheers,
Vicente.