Properly passing on horiz-/vertOn to Cursor()

Simple fix to properly pass on the horizOn and vertOn keyword
arguments to the matplotlib.widgets.Cursor constructor.
Make arrow docstring mention data transform
Autoscaling and limits in mplot3d.

    * Fixes improper setting of x,y,z limits not turning off autoscaling
    * Adds units support to z-axis
    * Adds shared axis support for z-axis
    * Closes #783
A bit of cleanup
backend_bases. Don't ignore the initial bbox when using bbox_extra_artist. Rather join the two bboxes
image comparison decorator: add option to pass kwargs to savefig
If a table exists add it to default_bbox_extra_artists. This avoids clipping the
table if bbox_inches='tight' is used.
Tables: Fix get_window_extent for table to allow table to be added to bbox_extra_artists.
The get_window_extent is a method on the cells not the key of the cells.
add documentation for savefig_kwarg
add test of bbox_inches='tight'
Add the bbox tests to default tests
address comments from pelson
_tri.cpp: Add explicit narrowing casts, to avoid C++11 compiler error.
_path.cpp: Add explicit narrowing casts, to avoid C++11 compiler error.
Remove all mention of make.osx in README.osx
savefig_kwarg default to empty dict not none to get rid of not needed if statment
savefig kwarg don't use a mutable type as a default argument
Allow Animation.save() to accept keyword arguments which are passed on to savefig() when saving individual frames. This can be used to set tight bounding boxes etc.
Don't use a mutable type (here, a dict) as default argument because it only gets evaluated once when the function is defined, not during each function call (i.e. changes will persist across function calls).
JRE - Modified 'use' so that it will only warn if the requested backend is different that what is currently set.