Coerce elements to strings as fallback

Returned objects from the APIs come back as `int`s in some cases. For example:

```python
from twython import Twython
import random

tw = Twython(...)
followers = tw.get_followers_ids()['ids']
sample_users = random.sample(followers, 99)
user_data = tw.lookup_user(user_id=sample_users) # Will result in a type error
```

One workaround is to map these ahead of time:
```python
tw.lookup_user(user_id=map(str,lonies))
```

This pull request just bakes it right in. We could coerce by default (which wouldn't affect strings).