Get statistics from last campaign

Prior to this PR, the `hubot-mailchimp` package could add and remove users from a given mailing list. This PR adds an additional command, simply called `mailchimp` that will return statistics for the last campaign that has been sent.

Fixes #3.

Demo:

```
User> hubot mailchimp
Hubot> Last campaign "Super Spring Sportswear Sale!" was sent to 255 subscribers (154 opened, 95 clicked, 10 unsubscribed)
```