Compara orthologs

This follows up #122 and contains things that don't have to go the live site
I have included #122 's two commits because the 3 following commits are based on them.
The diff of the 3 additional commits can be seen here: https://github.com/muffato/ensembl-webcode/compare/compara_ortholog_master...compara_ortholog

dd4bd45 and 935f7fa are just about removing unused code.
340fd1f is more invasive. To make the page that displays the alignment of two homologs faster, we can pass the dbID in the URL. fetch_by_dbID is much more straightforward than the previous scan (get all the homologs, and find the one with the expected target gene).
The code is still able to understand the former "g1" argument to define the target gene. I guess this is needed for compatibility with the archives ?