Tutorial for Finite Difference of Derivatives
Ran stripwhitespace on finite_diff_derivatives.rst given recent merge

This is a commit of the file after running stripwhitespace, given Chris Smith's
recent enabling of usage of stripwhitespace compatibility with .rst files.
Fix of some further typos
Fix of some suggested typos and code changes
Some fixes of typos...
Stripped whitespace again as a check
Some more prose changes
Remove whitespace and add some commas
Made new directory special_topics, moved finite_diff_derivatives.rst to special_topics dir, etc

After some discussion I have made a new subdirectory src/special_topics to contain longer and perhaps not at tutorial
level.  I also put src/special_topics/finite_diff_derivatives.rst,a new index.rst and a new intro.rst in the new
directory. I have edited src/tutorial/index.rst to reflect the content change.  And I modified the src/index.rst
to contain a reference to SymPy Special Topics.
Further small edits and clarifications
Changed print to print() and xrange to range for compatibilty with Python 3
Removed the currently empty section header on Forberg's method for derivatives
Fixed some problems with using print() and pprint.
Fix examples
Further attempts to fix problems with using print, pprint solved.
Cleaned up usage of print, removed pprint. Now hopefully done.
Repaired missing outputs of print() statements. Now all examples run correctly.
added sampling_density to sympy.stats
Add Benjamin Gudehus to AUTHORS/aboutus. Welcome to SymPy!
make sampling_density tests more robust