Added NJ negative branch calculation 

Added a function that recalculates the negative branch lengths as proposed by Kuhner & Felsenstein (1994).
Commented NJ modification

NJ modification commented to pass check. Code with no space after the pound sign is the actual code. Pound signs with spaces are real comments.
Moved NJ_negative_branch to interal.r
Created function fix_negative_branch to normalize negative branch lenghts. See Kuhner and Felsestein (1994).
Changed entire negative branch length logic. Using fix_negative_branch
Added bug fix to news
Merge branch 'master' into devel