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.
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Added methods to control theta position of r-ticklabels on polar plots
Merge pull request #2550 from joferkington/fix-wx-singleshot-timer

Using a single-shot timer with the Wx backend raises an AttributeError
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()
add titleweight rc param
actually read the new rcParam
reference rcparam in docstring for pyplot interface
fix pep8 errors.
MultiCursor: make events connected during __init__ accessible (for later removal)
fix initialization of AnnotationBbox where None values of xybox and boxcoords parameters are not correctly handled.
AffineBase.__eq__ should not raise an exception when the other does not have "is_affine" attribute.
Merge pull request #2581 from leejjoon/affine_base_eq

AffineBase.__eq__ should not raise an exception when the other does not ...
make event connector ids private