Bugfix: taxon names in the gene-tree displays

Hi !

If you have a look at the gene tree http://www.ensembl.org/Homo_sapiens/Gene/Compara_Tree?db=core;g=ENSG00000073910;r=13:32605437-32870794 you'll see ": 2 homologs" on its first line, whilst other nodes have a taxon name in front of the colon.

The bug is related to the changes in the SpeciesTree API that we've made in e74.
This commit fixes 3 things:
 - There is a name in $speciesTreeNode->node_name
 - We only need to fill the $self->db_tree->{$db_name}{'TAXON_NAME'} hash with common names to override $speciesTreeNode->node_name. The common name are stored internally as "ensembl alias name"
 - The webcode doesn't need to get NCBITaxon objects as the names are either in the $speciesTreeNode object, or in the packed config

With the commit, the tree looks like http://enssand-01.internal.sanger.ac.uk:9073/Homo_sapiens/Gene/Compara_Tree?db=core;g=ENSG00000073910;r=13:32605437-32870794