Adding dictionary-like interface to the Article object

I find it useful to have access to the Article attributes as if it's a dictionary. This pull request enables the following kinds of queries:

```
papers = list(ads.query(...))
print(papers[0].keys())
print(papers[0]["title"], papers[0].title)
for k, v in papers[0].iteritems():
    ...
```

What do you think of this?