Add scheduled query precompute functionality

The Jia UI now provides a timeframe interface, where the user specifies either a recent timeframe:
![screen shot 2014-07-15 at 11 06 58 am](https://cloud.githubusercontent.com/assets/1996521/3586576/ba488dce-0c31-11e4-9967-b29e3dad75f6.png)

or a specific date range:
![screen shot 2014-07-15 at 11 07 16 am](https://cloud.githubusercontent.com/assets/1996521/3586577/ba48ca1e-0c31-11e4-9c6f-909f742ace17.png)

The `start_time` and `end_time` variables are automatically injected into the scope of the query code blob (along with `events` and `kronos_client`).

When the precompute switch is activated, the results of the query code blob will be cached. When the query is over a recent timeframe, additional options are provided to the user so that the precomputation will happen on a schedule. This ensures the cache is always fresh when a user runs the query. When the query is over a specific date range, the precomputation is done only once.
It used to start automatically, but we decided against that because we aren't sure how Jia will be started (uWSGI?) and there can only be one scheduler instance. So it must be started manually.