Allow users to import the module and access item metadata without having to supply IA credentials.

Only require require cookies/keys if users need them. so allow them to import the module and download files without keys:
```
>>> import archive
>>> item = archive.Item('TestUpload_20130502_2234') #downloads metadata w/o keys
>>> item.upload('MANIFEST.in')
NoAuthHandlerFound: No handler was ready to authenticate. 1 handlers were checked.

>>> import os; 
>>> os.environ['AWS_ACCESS_KEY_ID']='x'; os.environ['AWS_SECRET_ACCESS_KEY']='y'
>>> item.upload('MANIFEST.in')
True
```