BUG: automatic color cycling when plotting Series without style or color argument (fixes #2816)
It should fix #2816 (and #2827 which is a duplicate).

Could maybe @wesm or @changhiskhan have a quick look at this one? I seem to remember the promise this would be fixed for the next release on the mailing list.

More than happy to change anything as needed.

Also I seem to have enabled travis-ci after pushing this branch to my fork so that this branch was never tested. If that makes things easier and there is an easy way  to force travis-ci to run on this branch let me know.
 There's a failing test:

```
λ nosetests ./pandas/tseries/tests/test_plotting.py:TestTSPlot.test_secondary_legend

======================================================================
FAIL: test_secondary_legend (pandas.tseries.tests.test_plotting.TestTSPlot)
----------------------------------------------------------------------
Traceback (most recent call last):
  File "/home/user1/src/pandas/pandas/tseries/tests/test_plotting.py", line 832, in test_secondary_legend
    self.assert_(len(colors) == 4)
AssertionError: False is not true
```

If it's fixed, I'll merge this.
 Hmm I thought I ran the tests and checked but apparently not. I'll take a look.
 Plot tests are usualy skipped due to slowness, you have to ask for them.

They were added to travis just a few days ago (tacked on to the LOCALE_OVERRIDE build).
or you can just do:

```
nosetests ./pandas/tseries/tests/test_plotting.py
```
 To be honest, not sure whether that's the best way to do it but the problem was that the left and right axis have independent color cycles which matters when you plot timeseries without any color or style arguments. In the failing test case you ended up having two blue and two green lines instead of the expected blue, green, red, cyan lines.

Any better work-around I am interested.
 Good enough for me.
Can you add a commit with a note in RELEASE.rst?
 This fix works great.
 merged
 great, thanks for this and sorting out the merge conflict.
