Use commands on the CLI: list configured image lists

- Add a new entry point "atrope".
- Add command module, with command handling.
- List configured image lists from the CLI.
command: fetch and verify image lists

Fetch and verify (signature and endorsers) all the image lists form the
command line
command: fetch only one image list

Fetch and verify just one image list
Tidy and clean image_list code
command: print if a list has a token configured or not
command: Handle our own exceptions on CLI

Do not print the traceback, but rather print an error to stderr
Print if list is verified and trusted and the reason
Add error information when we cannot fetch a list
Preserve traceback when re-rasing exceptions
bugfix in smime: raise unraised exception

There was an exception not raised if the lenght of the signers was equal
to 0.
PEP8 fixes
sync cache directory

Added a sync_cache method to the ImageListManager. This new method will
download and verify all the images from the configured image lists.
Also, added a command to trigger the sync.

- It will only take into account lists that are enabled, whose signature
  can be verified and whose endorser is trusted.
- For those image lists, the images are downloaded and the signature
  will be verified.
- Afterwards, it will clean the unknown files and old images from the
  directory.
handle exception if image checksum is incorrect

We re raise an exception if the image verification after the download
fails, and we capture the exception in the image list caching method so
that the invalid file is removed afterwards.
command: add image lists interactively from command line

It is now possible add an image list source from the command line, using
some guided instructions.

closes #4
command: handle KeyboardInterrupt

Exit nicely on KeyboardInterrupt
image lists: use empty strings as defaults

If some attributes were missing, they were set to None, instead of "".
It is beeter to use the empty string for those cases.
command: create image list non-interactively

Add the possibility to create an image list source non interactively.

closes #4