Fix observations counting

My gut feeling told me the observations counting is not correct, so I added an test and it turned out to be true:

```
Failures:

  1) Quantile::Estimator#observations returns the numer of recorded observations
     Failure/Error: estimator.observations.should == 42
       expected: 42
            got: 83 (using ==)
     # ./spec/estimator_spec.rb:39:in `block (3 levels) in <top (required)>'
```

While the easy fix would be to move the `@oberservations` increment our of the `#record` method, ideally we wouldn't need to create all these `Sample` values for performance reasons.