dispatchers: handle correctly VO and private images

Image lists protected by a token must be kept private. Moreover, if an
image has a VO associated to them, that VO should be able to have access
to those images.

closes #10
dispatchers: remove unused prefix flag
dispatchers: set prefix from image source

Now it is possible to set the prefix via the image list source.

Closes #15
doc: change readme to Markdown and add more information
doc: added sample configuration files

Closes #12
commands: add support for setting the prefix in the CLI

change f2c626f introduced support for setting the prefix for each of the
image lists. This change makes possible to set it using the CLI.
image list: Catch IOError exception

Catch the IOError exception and reraise our own exceptio so that the
traceback does not appear in the output.
start using an abc for the image list sources
move hepix image source to hepix.py module

The class ImageListSource is too dependent on the HePIX image list,
therefore it makes sense to have all of them in the same module.
fix missing prefix when writing image source
list manager: move common method to base class

The 'add_image_list_source' should be common to all the classes that
inherit from the base manager.
image source: pass extra arguments as **kwargs

We are passing the arguments for creating an image list source when
reading the yaml directly as kwargs. However, from the manager we do not
know what fields are required to create an image list source, therefore
we pass only the arguments we know and then we pass the rest in the
kwargs dict.

closes #14
rename and move YAML file into hepix module

The current implementation only reads YAML files related with HEPiX VM
image lists, so it makes sense to move the configuration option into the
hepix module.
glance: catch 409 Conflict when tenant is a member

When adding a member to the image, if the tenant is already a member, a
409 error (Conflict) was raised. Now we catch the exception and throw it
away.