Graphite x-range as status check model field

We have some stats that are populated once every minute with a very high resolution, and others updated once every 24 hours with a much lower resolution.  There's no common `GRAPHITE_FROM` that will produce useful graphs for both sets of statistics.
                                                                                
I made these changes:                                                           
- Added `x_range` as a status check field                                       
- Renamed `mins_to_check` to `points_to_check`                                  
- `points_to_check` is now a status check field                                 
                                                                                
The defaults (10 minute `x_range`, 5 `points_to_check`) should match the defaults in an unmodified Cabot installation.