Change the organization of %registry_register slightly to make it easier

to merge copies of it; added a _SPECIES and a _TYPE section to the hash.

Added the subroutine load_registry_from_multiple_dbs():

   Arg [1]   : Array of hashes, each hash being a set of arguments to
               load_registry_from_db() (see above).

   Example   :

     $registry->load_registry_from_multiple_dbs( {
         '-host'    => 'ensembldb.ensembl.org',
         '-user'    => 'anonymous',
         '-verbose' => '1'
       },
       {
         '-host'     => 'server.example.com',
         '-user'     => 'anonymouse',
         '-password' => 'cheese',
         '-verbose'  => '1'
       } );

   Description:  Will call load_registry_from_db() (see above)
                 multiple times and merge the resulting registries
                 into one, effectively allowing a user to connect to
                 databases on multiple database servers from within
                 one program.

                 If a database is found on more than one server, the
                 first found instance of that database will be used.