Add archive.File class and test cases

Add a file class:
```
>>> item = archive.Item('TestUpload_20130502_2234')
>>> item.file('sita_biohazard_56.55.33.png')
<archive.archive.File at 0x29c7ad0>
```

Files can be downloaded:
```
>>> f = item.file('sita_biohazard_56.55.33.png')
>>> f.download() #writes to disk
>>> f.download('/foo/bar/some_other_name.png')
```

You can iterate over files:
```
>>> for f in item.files():
…        print f.name, f.sha1
```

You can also run a minimal test suite by running `py.test`