Add support to swarming.py collect to stream stdout

Right now swarming.py collect only dumps the task stdout at once at the end of 
the task. There's no reason to not stream it as it's happening, since the 
server has all the data; the bot already streams the stdout to the server.

Action items:
This requires two changes;
- The server needs to support fetching the output incrementally. There's 
already a dedicated URL to fetch a single task output but it doesn't support 
content range or cursor-like fetching.
- The client needs to use this new logic.

Original issue reported on code.google.com by maruel@chromium.org on 20 Oct 2014 at 6:36

Blocked on: #179

madecoste / swarming

Add support to swarming.py collect to stream stdout #167