Added legend support to kdeplot()

Added a legend keyword to kdeplot() so that users can toggle legends
when displaying, e.g., multiple densities on the same plot. Labels do
not override the legend setting, so you need to assign labels and set
legend to "True" in order to see the legend as expected.
Added example of the legend keyword in kdeplot.

I added an example demonstrating how to use the legend keyword with
kdeplot. I also checked all of the distplot examples to make sure that
there are no side effects. I also tested the example at cell 21 ("To
control any of the underlying plots, pass keyword arguments to the
[plot]_kws argument.") using values of True and False for the displot
legend keyword. No problems were observed, and I set it back to False
to match the original notebook.
Assign lable name for legends using DataFrame or Series name

Check for a name attribute on the input and automatically set that as
the label if it exists for Pandas DataFrames and Series. If the user
specifies a label in the call to kdeplot it will take precedence over
the name attribute. If no label is provided and there is no name
attribute then a default value of "kde" is assogned.
Changed to duck typing for name detection

If the input data has a name attribute, use it for a label if one is
not slready set.
More work on kdeplot and legends

Did more work on legend handling in kdeplot.

Made changes to label and legend handling in distplot. added keyword
kde_label (defaults to False). If it's set to True, then
kde_kws["label"] is set to "kde", and the KDE in the resulting plot
will be labeled "kde".

Added more complete description of how to use kdeplot labels to
notebook. Added  example of vertical kdeplots.
Refined version of vertical kdeplot.

Tweaked sample vertical distplot in cell 17.
Removed kde_label argument to distplot.

Also added example of how to use kde_kws arguments to control labeling
of the kernel density estimate in distplot().
Indentation fixes.
Merge branch 'master' of github.com:wintermind/seaborn into wintermind-master
Finalizing label additions to kdeplot
Updating demonstrations of kde label functionality in dist notebook
Simplifying distributions notebook