Classification report digits test

Added digits as optional argument for classification report (per issue #3749), and added unit test to test_classification.py. Also cleaned up so that merge is on top of master. Changes pass clean through nosetests --with-coverage, etc.
formatting is not correct.

```
digits : int
    number of digits for formatting output floating point values.
```

but I am wondering if using a format attribute such as fmt param in np.savetxt wouldn't be more explicit/standard.