added get_user_info call

Ability to query user info via users/me.

Added as __request is private, perhaps this shouldn't be a private method?

Note: Haven't added tests for this, simple enough call - but haven't had time to dig into tests.py

Here's a fixture from the box docs.

```
{
    "type": "user",
    "id": "17738362",
    "name": "sean rose",
    "login": "sean@box.com",
    "created_at": "2012-03-26T15:43:07-07:00",
    "modified_at": "2012-12-12T11:34:29-08:00",
    "language": "en",
    "space_amount": 5368709120,
    "space_used": 2377016,
    "max_upload_size": 262144000,
    "status": "active",
    "job_title": "Employee",
    "phone": "5555555555",
    "address": "555 Office Drive",
    "avatar_url": "https://app.box.com/api/avatar/large/17738362"
}
```