Allow fractional interval argument for database worker

[mgax]

Running the database worker with a fractional interval fails:

$ ./manage.py db_worker --interval=.001
...
manage.py db_worker: error: argument --interval: invalid valid_interval value: '.001'

There's a built-in way to check if a float value is an actual number (as opposed to NaN or Inf) which is math.isfinite.

[mgax]

I think the tests are failing because branch is out of sync. Let me check.