BUG graphics influence plot, switched criterion

[josef-pkt]

closes #3103

I added a unit test to compare the sizes used in the plot, but it uses a matplotlib figure private attribute _sizes. This is most likely not robust, and might need to be try except AttributeError protected

I used matplotlib '1.5.1', and tests pass on my computer (only for the test method, I still get segfaults running the test class)

Detail (as explanation to test code): the plotted sizes are a affine transformation of the influence criterion, so I check that the residuals of the linear regression are close to zero.

[josef-pkt]

the test doesn't seem to work with any of the matplotlib used on travis, i.e. <1.5

possible problem with try...except catching is that the unit test might never run at all.
(We need more robust unit tests for content of plots)

[josef-pkt]

maintenance, some test methods create several plots but don't close them
If I close all of them immediately, then I don't get a segfault anymore when running the unit tests.

https://travis-ci.org/statsmodels/statsmodels/jobs/152251848

statsmodels.graphics.tests.test_regressionplots.TestPlotPandas.test_plot_oth ... /home/travis/miniconda2/envs/statsmodels-test/lib/python2.7/site-packages/matplotlib/pyplot.py:412: RuntimeWarning: More than 20 figures have been opened. Figures created through the pyplot interface (`matplotlib.pyplot.figure`) are retained until explicitly closed and may consume too much memory. (To control this warning, see the rcParam `figure.max_num_figures`).
  max_open_warning, RuntimeWarning)
ok

I still get tkinter warnings in TestPlot.test_plot_oth unless I comment out the add_lowess loop. I still don't see where this comes from.

statsmodels.graphics.tests.test_regressionplots.TestPlot.test_plot_oth ... can't
 invoke "event" command: application has been destroyed
    while executing
"event generate $w <<ThemeChanged>>"
    (procedure "ttk::ThemeChanged" line 6)
    invoked from within
"ttk::ThemeChanged"
can't invoke "event" command: application has been destroyed
    while executing
"event generate $w <<ThemeChanged>>"
    (procedure "ttk::ThemeChanged" line 6)
    invoked from within
"ttk::ThemeChanged"
can't invoke "event" command: application has been destroyed
    while executing
"event generate $w <<ThemeChanged>>"
    (procedure "ttk::ThemeChanged" line 6)
    invoked from within
"ttk::ThemeChanged"
ok

[coveralls]

Coverage increased (+9.0e-05%) to 89.184% when pulling 2f54d07 on josef-pkt:bug_influence_plot_3103 into 92da1cd on statsmodels:master.

[josef-pkt]

test_power is the last one, the entire test suite runs now for me also.
quite a bit faster than Travis 516.624 / 60 = 8.6104 minutes
(single core nosetests, but MKL uses multicore, max cpu usage around 50% with 4 core/8 logical processors when running the tests for statsmodels.regression)

Ran 4890 tests in 516.624s
FAILED (SKIP=35, errors=2)

(second error is in an extra file, that was left over from a different branch)

[josef-pkt]

some cosmetic changes would be possible, but in it goes