Python-nexus

- added rudimentary tree handling to `NexusWriter` objects:
python
>>> nex = NexusWriter()
>>> nex.trees.append("tree tree1 = (a,b);")

- added the ability to combine nexuses containing trees

- remove some over-engineered checking on the `NexusReader.DataMatrix.characters` property
- major refactoring of reader.py into a `handlers` subpackage
- `NexusReader.read_string` now returns self, such that it can be used as a factory-style method.
- added rudimentary support for taxon annotations in taxa blocks.

- work around a minor bug in BEAST2 (https://github.com/CompEvol/beast2/issues/713)
- `characters` block is now added as `characters` and not invisibly renamed to `data`.