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
Hi @ev-br 

  * the values of x and y come from the example listed in the docstring for the class UnivariateSpline, but shortened to a length of 10 elements
```
>>> x = linspace(-3, 3, 100)
>>> y = exp(-x**2) + randn(100)/10
```
  * x_out_of_range is an extension of x_in_range via the following code
```
>>> x_out_of_range = linspace(-11/3., 11/3., 12)
```
  * the value of `desired` is obtained via UnivariateSpline on x_in_range and y and prepending and appending zeros. This is meant to assert near-equality in floating point, as you mentioned.
  * This should work with a smaller tolerance, I just chose 5e-4 based on the tolerance that I saw in another test. Should I decrease the tolerance?