Check the response status when getting the list
Use objects for the lists

Closes #1
Do basic authentication using OAuth token.
Rename image_list objects:

- List to ImageList
- ImageLists to ImageListManager
Initial download of images

When a list is loaded, create Image objects for each of the images that
the list contains. Afterwards we're able to download them.
Move the download method to the image
Make image.Image class abstract
Verify image's checksum against list's checksum
Improve image download and verification

- do not download again images that are valid
- improve logging