Add documentation

Resolves #19.
Added the necessary information to the README. The idea is to let a new user take a quick look and understand how to use the API.

I should've been clearer / I should update the spec:

When designing it I thought these should return an array of these values.
Given that most of the uses for an aggregate view would presumably be for data viz, they probably will want more than one specific window. This might not be a good assumption. Thoughts?


```
{
    "next_start_time: "2014-02-28"
    "group_id": "butler-3",
    "count": 100,
    "results": [
        {
            "start_time": "2014-03-28",
            "average": 10,
            "minimum": 0,
            "maximum": 100
        },
        {
            "start_time": "2014-03-27",
            "average": 10,
            "minimum": 0,
            "maximum": 100
        },
        ....
    ]
}
```