matplotlib Code

Make mlab.psd() call mlab.csd() instead of duplicating 95% of the code. Tweak csd() to check if x and y are the same and avoid duplicating the work if so.

Update the axes.csd() to match the new options added to mlab.csd(). Move some of the docs from axes.psd() to the common place in mlab.

Update the mlab.csd() to match the new options added to mlab.psd(). Factor out the keyword argument docs so that they can be shared between the two functions.

Slight whitespace cleanup.

Update the axes.psd() method to reflect the new options available in mlab.psd. Add an example to show how the options change things.

Add 'pad_to' and 'sides' parameters to mlab.psd() to allow controlling of zero padding and returning of negative frequency components, respecitively. These are added in a way that does not change the API.

Fix autoscaling problem caused by axhline and axvline

Fix handling of c kwarg in scatter, with array of strings;
modification of patch by Ryan May. The fix involves making
cbook.is_string_like handle variables of type numpy.string_.

Clean up some of axes.psd() docstring. Add a reference to the psd_demo.py example.

Make iterable() and is_string_like() return True/False instead of 1/0.