ENH: Panel.shift now uses NDFrame.shift.
PRF: Adding Panels.shift test

DOC: Panel.shift no longer drops periods.
Merge branch 'general-shift-4867-panel' of https://github.com/dalejung/pandas into dalejung-general-shift-4867-panel

Conflicts:
	doc/source/release.rst
	doc/source/v0.14.0.txt