add biweight midcorrelation as option for series correlation
Add [Biweight Midcorrelation](http://download-v2.springer.com/static/pdf/95/chp%253A10.1007%252F978-3-642-39678-6_14.pdf?token2=exp=1428360548~acl=%2Fstatic%2Fpdf%2F95%2Fchp%25253A10.1007%25252F978-3-642-39678-6_14.pdf*~hmac=980c2edddba12bbb73ebbadaa682092cde9d534bc8738d65785647402b683c21), an alternative correlation method to Pearson that is robust to outliers, to the series correlation method repertoire.
