endorser: add endorser object
image: refactor image class

- rename class to HepixImage
- include a map of the required fields into object attributes.
image lists: refactor image list objects

- add a new HepixImageList class that will hold all the Hepix Image
  List stuff.
- Refactor ImageList object into ImageListSource and adapt to the new
  HepixImageList object.
image lists: remove images from expired lists

closes #6
image lists: subscribe only to some images

closes #3
image lists: actually remove the invalid files

Fix a bug that removed the latest basedir, instead of the invalid files