validate aggregation field in report config

![screen shot 2014-12-19 at 4 15 57 pm](https://cloud.githubusercontent.com/assets/1757035/5511347/5a423efe-879a-11e4-86e5-a15bc5069a0e.png)

I think you might be able to simplify this by just adding `choices=SQLAGG_COLUMN_MAP.keys()` to the `aggregation` property initialization.

also a great candidate for quick and easy tests! e.g. https://github.com/dimagi/commcare-hq/blob/master/corehq/apps/userreports/tests/test_report_filters.py#L18