[NEW]: support attachments from file or url

Hi!

I needed to upload attachments, so I went ahead and added that. I tested the code as follows:

```python
with open("/path/to/file.png", "rb") as f:
    card.attach(name="Another test.png", file=f)

card.attach(url="http://i.imgur.com/A8kVJsJ.gif")
```

The tests file state is broken, so I didn't want to add anything there till that is fixed. With that said, I'd be happy to join in and contribute on my free time. Things like reorganizing the tests, reducing duplicate code, and having a more consistent API.

Thanks,
Maz