Allow Monitors to use Validators

I'd like to be able to validate monitored values as they are read. I don't think this is possible because the `monitors()` function doesn't allow you to pass a `Measurement` object.

Example:
```
@monitors(Measurement('current').in_range(0.2, 0.5), poll_Interval_ms=50)
def turn_on(test):
do_turn_on()
wait_for_init(seconds=1)
```