added 2 new methods: get db_name for an external_db_id and fetch db_entries for a source (db_name)
added 2 new methods: get db_name for an external_db_id and fetch db_entries for a source (db_name)

at7/ensembl,muffato/ensembl,james-monkeyshines/ensembl,muffato/ensembl,james-monkeyshines/ensembl,at7/ensembl,james-monkeyshines/ensembl,at7/ensembl,willmclaren/ensembl,muffato/ensembl,Ensembl/ensembl,muffato/ensembl,willmclaren/ensembl,Ensembl/ensembl,Ensembl/ensembl,danstaines/ensembl,mjg17/ensembl,danstaines/ensembl,at7/ensembl,mjg17/ensembl,danstaines/ensembl,james-monkeyshines/ensembl,mjg17/ensembl,mjg17/ensembl,Ensembl/ensembl