Index to better support downloaders

Hi Karsten, as discussed at the dev meeting it would be sweeeeeet for Stem to have a CollecTorDownloader class that supports getting and processing descriptors from CollecTor.

Presently you read and process the Apache index html, but it would it could be a lot nicer to have a machine readable index.json file instead. This is the ticket for it.

To support future expansion it would be nice to be able to include additional metadata, so I propose a 'contents' hash, such as...

https://collector.torproject.org/recent/index.json

{
  'resource': 'https://collector.torproject.org/recent/index.json',
  'contents': [
    {
      'name': 'bridge-descriptors',
      'type': 'directory',
      'last_modified': '31-May-2014 09:49',  # or a unix timestam?
      'url': 'https://collector.torproject.org/recent/bridge-descriptors/'
    },
    {
       'name': 'exit-lists',
       'type': 'directory',
       'last_modified': '11-Oct-2015 15:02',
       'url': 'https://collector.torproject.org/recent/exit-lists/'
    },
    {
        'name': 'example-file.txt',
        'type': 'file',
        'last_modified': '31-May-2014 09:50',
        'size': 42901,
        'url': 'https://collector.torproject.org/recent/example-file.txt'
    }
  ]
}