Changes to the in development PhyloXML IO

Deleted old code from treeio.jl & started new file to contain phyloxml
code whilst it is developed. Started new XML parsing code using
LibExpat, finished the CladeTaxonomy type and the code to read in
Taxonomy information for a clade element in PhyloXML format files.
Started the code for reading in Sequence information for a clade
element in PhyloXML format files.