Fix /mapreduce/pipeline to properly ZipImport

Test Plan:
Craig's review
(ZipImport's only happen in production, so hard to test locally)

Reviewers: csilvers

Differential Revision: http://phabricator.khanacademy.org/D2281
Merge branch 'master' of https://github.com/Khan/appengine-mapreduce
Truncates parameters and output values for the pipeline Web UI so it won't crash on large pipelines.

Test Plan:
1. Start a big pipeline with the following config at /devadmin/studentusedata

{
    "activity_start_date": "2012-08-01",
    "activity_end_date": "2013-05-31",
    "in_class_time": {
        "timezone": "US/Pacific",
        "days": [1, 2, 3, 4, 5],
        "times": [
            ["08:00:00", "15:30:00"]
        ]
    },
    "coach_ids": [
        "sal@khanacademy.org"
    ]
}

2. After the page refreshes, click on the pipeline ID to open the pipeline UI
3. Click around on several of hte child pipelines

Notice that no "Error: JSON data could not be decoded" messages are thrown