Introducing 'info' request

[loerum]

Implementing a "info" request to a move it server. For a request with topic "topic"
and type "info" the server will reply with a list of published files belonging to
topics starting with "topic".

Notes (and room for improvements):

• Server will keep cache of published files (like client).
• Currently there is some (arbitrary) limit on cache size and amount of data to be send.
• The cache is not persistent. This is kind of OK, since a server on start up, will publish
  files in "origin" (not quite correct, the original order of published files are lost).
• Introducing bin/move_it_req.py to send request (ping or info) to a server.
• No time stamp on published files, so an info request can only specify a max count of files
  (not what has been published since yesterday at 12).

[mraspaud]

@loerum lgtm, can I merge this ?