[Bug 1091268] API to generate users.

The lists here aren't final, I'm waiting for NI on the bug. What do you think of the general idea though? This generally works like:

```bash
$ http POST http://kitsune/api/2/user/generate
<headers>

{
    "password": "IhkUvecGas",
    "token": "6b58d402579e8a409b3a3de0d72880bb3b36295a",
    "user": {
        "avatar": "https://secure.gravatar.com/avatar/d41d8cd98f00b204e9800998ecf8427e?s=48",
        "bio": null,
        "city": null,
        "country": null,
        "date_joined": "2014-10-30T11:51:53.261",
        "display_name": null,
        "facebook": null,
        "irc_handle": null,
        "locale": "en-US",
        "timezone": "US/Pacific",
        "twitter": null,
        "username": "HappyGreenCat",
        "website": null
    }
}
```

The avatar is obviously bogus, since there is no email. I'd like to someday fix that and let us have a real hybrid avatar system, but for now these uses get the default avatar.

f?
The docs for DRF will mention a better way to do this. That better way is is in DRF 2.4 though, which we don't have.