New method load() for Exon, Gene, Transcript, and Translation.

=head2 load

  Arg [1]       : Boolean $load_xrefs
                  Load (or don't load) xrefs.  Default is to load xrefs.
  Example       : $translation->load();
  Description   : The Ensembl API makes extensive use of
                  lazy-loading.  Under some circumstances (e.g.,
                  when copying genes between databases), all data of
                  an object needs to be fully loaded.  This method
                  loads the parts of the object that are usually
                  lazy-loaded.
  Returns       : Nothing.

=cut