Misc improvements

This fixes a few things.

First, it makes response objects do something reasonable if you drop them in the debugger:

```
(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)
```

Then, I pulled the target normalization stuff out into a helper, because for some reason I thought it was going to be more guttable (my original patch kinda sucked). Happy to just inline this back into the original method now though if you like, the util module looks kind of lonely :(

And then I fixed some random pyflakes warnings while I was thinking.