matplotlib Code

added more examples to backend driver

add support for docstring.hardcopy to format the docstrings less verbosesly for interactive use

removed dashtick

fixed some doc formatting errors

Replace tabs with spaces.

fixed some doc formatting errors

Remove trailing whitespace.

Update CHANGELOG and api_changes for the changes to psd and friends.

Add a converted MatLab example that was used to figure out the differences between MatLab and matplotlib PSD scaling.

Update spectral methods (psd, csd, etc.) to scale one-sided densities by a factor of 2 and, optionally, scale all densities by the sampling frequency. This gives better MatLab compatibility. Update corresponding Axes methods, and make Axes.psd() label the y-axis of the plot as appropriate.