Carbon shorthand

An attempt at support carbon-style shorthand retentions (issue #2) with some tests. I've tried to match the code-style wherever possible. Allows you to specify the following in your `cyanite.yaml`:

```yaml
carbon:
  rollups:
    - "15s:6h"
    - "1m:3d"
    - ...
```

The strings are converted into standard `:period` `:rollup` pairs before going through the process they would have already gone through.

I think the logic is correct. I had to figure out how it all worked to get past my own stupidity in issue #19) and thought I should help someone else make the same mistake I did when migrating from carbon.