ENH: Add ext parameter to UnivariateSpline call

Pass parameter to fitpack.splev to control how to handle out of range
evaluations. Added test to scipy/interpolate/tests/test_fitpack2.py.

See ticket 3557.

closes gh-3557
Ah, I see now. I think the test should be rewritten to avoid the random noise: you're testing correctness of the extrapolation, not the smoothing of the noise.
I'd just do something as simple as of
```
>>> x = np.arange(5, dtype=np.float)
>>> y = x**3
>>> spl = UnivariateSpline(x, y)
>>> xp = np.linspace(-8, 13, 100)
>>> assert_allclose(spl(xp), xp**3, atol=1e-16)    # current behavior
>>> assert_allclose(spl(xp, ext=0), xp**3, atol=1e-16)
>>> assert_allclose(spl(xp, ext=1), 0, atol=1e-16)
>>> assert_raises(ValueError, spl, xp, 0, 3) 
```


A further (really tiny) nitpick is to convert the docstring of the test case into a comment: perhaps counter intuitively, the preference in scipy is to _not_ have docstrings for test functions.