Replace Tornado with asyncio and aiohttp

Replace Tornado with asyncio and aiohttp for our async and http client
needs:
    - aiohttp has a better API than Tornado's httpclients, allowing the
      long-polling code to be simplified.
    - aiohttp supports keep-alive, which will reduce long-polling
      disconnections (fixes #15).
    - asyncio will make it easier for other projects to use hangups.

hangups now requires a development version of urwid with asyncio
support.