Added time_shift() method to Evoked class.

This method extends the functionality of the evoked class by allowing a
time shift of the scale by a certain amount. The data is not touched, but
only the time-axis is changed.
Added unit test for checking time_shift() method in Evoked class.
Additionally, the time_shift() method itself was made more efficient by
unifying the absolute and relative conditions in one statement.
Changed assert_array_almost_equal() to np.allclose() for the data because
it is more robust to variability of scale.
pep8