vignettes moved to their own directory as suggested since R 2.14

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/phylobase@844 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
vignettes removed from inst/doc as suggested since R 2.14

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/phylobase@845 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
adding option convert.edge.length to readNCL to convert negative branch lengths to 0 so that objects can be imported into phylobase

git-svn-id: svn://svn.r-forge.r-project.org/svnroot/phylobase@846 edb9625f-4e0d-4859-8d74-9fd3b1da38cb