Change fetch_logs.py to download all logs from the new /api/v1/fetch_logs

Test Plan:
First, I deployed to znd-serverside-appcfg.

Then I ran these commands with both the old fetch_logs.py script (in the analytics repo) to verify that the output was correct.

```
python ./fetch_logs.py -s "2013-10-30T00:00:00Z" -e "2013-10-30T0:00:02Z" -i 1
python ./fetch_logs.py -s "2013-10-30T00:00:00Z" -e "2013-10-30T0:00:02Z" -i 1 -b
python ./fetch_logs.py -s "2013-10-30T00:00:00Z" -e "2013-10-30T0:00:02Z" -i 1 -v znd-serverside-appcfg
```

There are also some unit test included with this changeset.

Reviewers: jace, chris, james

Reviewed By: james

CC: ben

Differential Revision: http://phabricator.khanacademy.org/D4888