TST : tweak test_add_collection

This is needed because this test tries to add artists from one axes to
another which is not really supported.  Not 100% that this is fully
equivalent to the current test.  There is a call to `add_collection` in
scatter, but the test might be defeated by some of the other logic in
scatter.
MNT : fix `__init__` order in AnchoredSizeLocator

Trying to set the axes property before calling up the mro to the base
class (eventually Artist) `__init__` which means that the `_axes`
attribute is not yet attached to the instance object.

 - removed setting of `self.axes`, this is taken care of in the
   baseclass
 - moved rest of sub-class specific attributes to after call up mro
   stack
MNT : tweak how internals of Axes.plot work

The actual processing and artist creation in Axes.plot happens through
the `__call__` method on a _process_plot_var_args instance hanging off
the Axes object.  The _process_plot_var_args knows what axes it is
hanging off of and passed that into the Line2D artists as a kwarg at
creation time, which sets the axes of the Line2D.  The axes of the Line2D
is set again during the `Axes.add_line` method.  For the most part this
is fine, if un-needed, because the axes is the same objects both time
it is set.  However when using the mpl_toolkit, Axes objects get wrapped
in not-strictly-subclassed objects and the Axes object that the
_process_plot_var_args object knows about is not the Axes object that
add_line is called on which now results in an exception as it is moving
artists between axes.
API : property-ify Artist.axes

 - deprecate Artist.{get,set}_axes
 - add axes property
 - Raise exception when trying to move artists between axes.
Merge pull request #3835 from tacaswell/single_axes_artist

Single axes artist