Use a plot directive rather than a link to a file in the _image directory, since the latter doesn't work with the latex build
A number of small fixes to reduce warnings during doc build
Merge pull request #1896 from mdboom/doc-warnings

Remove some documentation build warnings
Merge pull request #1953 from mdboom/fix-doc-build

Fixed use of generated image in documentation build.
Add skew() and skew_deg() to Affine2D.

These set up skew transforms, which can be viewed as indpendantly
rotating the X and Y axes.
Fix Bbox transformation for skewed Axes.

This involves transforming the upper left and lower right points
separately to come up with the combined translated upper right.
Replace hand-creation of blended transforms in Axes.

By default get_[x|y]axis_transform() do the same, but by calling these
methods instead of creating the transforms by hand, we better support
custom Axes.
Remove unecessary use of blended transforms in spines.

The manual use of the blended transform breaks non-seperable custom
axes/projections. Removing this does not appear to affect any of the
spine examples.
Re-order drawing of spines and axis objects.

This gives the spine a chance to update the axis object before draw.
This is sensible given that the axis can be registered with the spine
so that it can modify it, and is necessary for some use cases.
Add skewt.py as api example.
Change skew() angle definitions.

Instead of specifying the angles as rotations of the axes, it is easier
(and more standard) to think of the angles in the skew transform as
shearing angles along each axis. Doing so means the X and Y angles
are swapped from how they were used before, so this commit also
updates the example.
Fixed variable names to be PEP8 compliant