Add support for advanced statistics

SendGrid has some [advanced statistics][api] that they allow you to pull from their Web API. This pull request adds support for the advanced statistics to the Statistics client.

It also has tests for parsing the JSON response fixtures I created based on the results of my own API calls. I didn't add any remote tests though, but can if you like.

[api]: http://sendgrid.com/docs/API_Reference/Web_API/Statistics/statistics_advanced.html