API to list members of a community. Resolves #2395 

Request `GET for/test-community/index.json`

returns:
```
{
    "members": [
        {
            "avatar": "https://avatars.githubusercontent.com/u/592286?s=128",
            "elsewhere": {
                "github": {
                    "id": 1,
                    "user_id": "592286",
                    "user_name": "ivanmushketyk"
                }
            },
            "giving": "0.00",
            "goal": null,
            "id": 1,
            "my_tip": "0.00",
            "npatrons": 0,
            "number": "singular",
            "on": "gittip",
            "receiving": "0.00",
            "username": "ivanmushketyk"
        },
        {
            "avatar": "https://pbs.twimg.com/profile_images/1108655589/_______.JPG",
            "elsewhere": {
                "twitter": {
                    "id": 8,
                    "user_id": "179171495",
                    "user_name": "IvanMushketyk"
                }
            },
            "giving": "0.00",
            "goal": null,
            "id": 8,
            "my_tip": "0.00",
            "npatrons": 0,
            "number": "singular",
            "on": "gittip",
            "receiving": "0.00",
            "username": "f8db672eb030L"
        }
    ],
    "name": "test_community",
    "slug": "test-community"
}
```