Issues #194 (cov function) and #92 (multi-column sort)
Added:
1. Series.cov
2. DataFrame.cov
3. test_cov in test_series.py
4. test_cov in test_frame.py

Modified:
1. DataFrame.sort
2. DataFrame.sort_index
3. test_sort in test_frame.py

In the future you should make these commits in a feature branch instead of master-- I'll have to cherry-pick these commits but then you should clean up your master branch to not have any merge commits-- after I take your commits you should reset to a state prior to this work then pull wesm/master...git-fu =) 
 You should also set your e-mail address in your global git config 
 OK I cherry picked and amended your commits and merged them in. Thanks a lot for the PR-- I'll make a few little edits and see if there's a more performant way to do multi-column sorting.

Also pls stick to 80 chars per column / other pep8 conventions
