Update Resource following changes in Connexions/cnx-epub#23

- Use `with resource.open() as f: f.read()` instead of `resource.data`
- Directly create `io.BytesIO` objects for Resource data
- Remove code to generate sha1 hash and use the cnxepub.Resource hash
Merge pull request #77 from Connexions/update-resource

Update Resource following changes in Connexions/cnx-epub#23