WIP: time_shift() method for Epochs class.

However, the unit test fails right now. Comments and reviews welcome!
No more bugs in shift_time_scale() method in Epochs class

The method name in the Evoked class has also been modified.
Corrected dashes in the docstring
Preloading of data not required. Ancient relic that must be discarded
Added example for time-shift and added time-shift in events.py
Dropped support for time-shift in Epochs class in favor of time-shift at the events level
Changelog modified conforming to the usual practice of appending newer changes to the bottom
Shorter name for time-shift in Evoked class
Small mistake in whats_new.rst rectified
MISC : cleanup time shift