ENH: Add ext parameter to UnivariateSpline

Pass parameter to fitpack.splev to control how to handle out of range
evaluations. Parameter ext can be passed via __init__ or __call__. Added
test to scipy/interpolate/tests/test_fitpack2.py. Tests if UnivariateSpline
call equals x ** 3 for out of range x and extrapolation modes 0 and 1.

See ticket 3557.

closes gh-3557