Added correlation(), moment(), cmoment() in rv_interface.py
Fixed whitespaces
Rewrote variance, cmoment and skewness in terms of moment
refactor variance and skewness with cmoment

This requires changing some tests around so that they are less
sensitive to the exact form.

I also had to skip a couple (doc)tests
Added smoment() and refactored skewness()
Added better test-cases, removed bad ones
Merge pull request #1875 from ankit-maverick/add_moment_and_cmoment_to_statistics.PDF

Added correlation(), moment(), cmoment() and smoment() in rv_interface.py