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
pep8 for backend_pdf.py
remove backend_pdf from pep8 test exclude list
minor code standard changes
minor code formatting
Fix download button
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
Merge pull request #2459 from megies/pep8_backend_pdf

pep8 for backend_pdf.py
BLD Fix build failure on Python 3.4b1 for Windows Windows

As of version 3.4b1 the official CPython distribution for Windows uses Tcl/Tk 8.6.
Merge pull request #2613 from cgohlke/patch-1

BLD Fix build failure on Python 3.4b1 for Windows
fixes issue #2482 and adds note about incompatibility between bbox options and animation backends
fixed a typo
fixed another typo
Merge pull request #2615 from vlsd/issue2482

Fixes issue #2482 and adds note in matplotlibrc.template
Refactor mechanism for saving files.
Move all print_ methods to their backends.
Manage default format-to-backend mappings in backend_bases.
print_figure() can now determine the backend for saving figures.
Remove print_ code duplication, add canonical FigureCanvas names.
Better documentation for print_figure refactoring.