envift does not choke on character variables but ignores them (with a warning)

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2809 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
cleaner handling of all non-numeric variables as factors in envfit

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2810 edb9625f-4e0d-4859-8d74-9fd3b1da38cb
vegan actually depends on R >= 2.15.0: paste0()

git-svn-id: svn+ssh://svn.r-forge.r-project.org/svnroot/vegan/pkg/vegan@2813 edb9625f-4e0d-4859-8d74-9fd3b1da38cb