OrthoXML support for homologies

Add a new output format for homologies (OrthoXML, with the content-type "text/x-orthoxml+xml")
I have added the "naughty but needs must" "text/x-orthoxml" content-type too (not sure why it is needed)

I have copied and adapted files from the gene-tree endpoints

Yes, this is missing tests, like all the gene-side of Compara, but I'd like to know if this part is fine