Impl __repr__ on api responses for debugging

Makes dumping response objects in the debugger more helpful; example:

```
(Pdb) res
<<class 'voltron.api.APIErrorResponse'>: success = False, error = True, body: {'status': u'error', 'message': u'An error occurred', 'code': 4096, 'type': u'response'}>
(Pdb)
```

Instead of:

```
(Pdb) res
<class 'voltron.api.APIErrorResponse'>
(Pdb)
```