ENH: Add makePeriodPanel
BUG: (GH4853) Fixed Panel.shift/tshift to use freq.
TST: Added Panel.tshift test case
RST: Added to release notes
CLN: on top of dale-jung 4863, remove shift from series.py, move to generic.py
DOC: v0.13.0 adds
Merge pull request #4864 from dalejung/panel-tshift-4853

Panel tshift 4853
CLN: clean up test_graphics.py
CLN: clean up tseries/tests/test_plotting.py
Merge pull request #4876 from cpcloud/plotting-test-cleanup

CLN: clean up test plotting