Fix bugs related to bottom kwarg in step histograms

Conflicts:

	lib/matplotlib/axes.py
	lib/matplotlib/tests/baseline_images/test_axes/hist_step.png
Consider ourselves interactive if sys.flags.interactive is set.
overloaded `_make_twin_axes` on `LocateableAxesBase` so that twinx and
twiny will work.
Add comment
Added methods to control theta position of r-ticklabels on polar plots
pep8 for backend_pdf.py
remove backend_pdf from pep8 test exclude list
minor code standard changes
minor code formatting
more informative exceptions for empty/not-existing images in
compare_images()
Fix download button
Merge pull request #2567 from megies/compare_image_exceptions

more informative exceptions for empty/not-existing images in compare_images()
Merge pull request #2549 from joferkington/add-rlabel-position

Add methods to control theta position of r-ticklabels on polar plots
Merge pull request #2409 from neggert/hist_bottom_fix2

Fix bugs related to bottom kwarg in step histograms
improve docstring and add test fot to_rgb(<float>)

Even if the docstring was saing it, emphasis that to_rgb/to_rgba do not
accept float as input, but string representation of a float.

As one might be tempted to had float->gray conversion add failing test
in case the functionality is added.

Side fixes :
    change tuple([value]*3) to (value,)*3 for speed

In [18]: %timeit  tuple([0.4]*3)
1000000 loops, best of 3: 505 ns per loop

In [19]: %timeit  (0.4,)*3
10000000 loops, best of 3: 23.5 ns per loop

In [20]:  (0.4,)*3 == tuple([0.4]*3)
Out[20]: True

Closes gh-2609
qt4 subplottool is modal now + new layout
subplottool -> whats_new.rst
fix wrong backend name in whats_new
cleanup in subplottool + new style qt fix

Signed-off-by: Thomas Hisch <t.hisch@gmail.com>
Merge pull request #2459 from megies/pep8_backend_pdf

pep8 for backend_pdf.py