nexml can contain multiple otus nodes!
extended this in most places, still a few tests to check (one ape test failing..)
also work on characters, not quite complete. More details to come in next commit
which should hopefully fix the remaining issues to be clean enough to push to
origin/master.  gotta run now though, so better commit than no commit
basic character matrix extraction implemented.
Provides the S4 definitions for character matrices (serializing and parsing the XML) for issue #12.
Provides basic test cases in test_characters.R.  Provides basic extraction of character matrix as discussed in #38.
Conversion of discrete states to their symbol values not yet implemented.  Still need to add support for multiple
states nodes corresponding to multiple characters, and dereferencing these appropriately