move handling of basic metadata out of write_nexml's explicit function definition and into seperate add_basic_metadata function
add method to get rdf from rdfa #20
add children to meta elements using the 'meta()' function
added documentation, handling of namespace additions (including avoiding duplicates)
handle the list/multiPhylo/phylo cases more elegantly
updated documentation
updated tests
add vignettes directory
updated ignore list
fix vignette dir name
check that get_characters returns same characters as written in, modulo the ordering
updated test suite to reflect recent changes.  All tests passing