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
This test is unclear:
* what are these values of x and y, and why should spl(x_out) equal `desired` (have you meant to assert near-equality in floating point?)
* a tolerance of 5e-4 does not look good really. 

A simple way of testing extrapolation is to use a linear function (eg gh-3697), or better use some known polynomial function.