Added dumping of files suitable for loading into target core database:
Added dumping of files suitable for loading into target core database:

 - xref.txt - contains xrefs that were mapped to e! objects during the mapping run
 - external_db.txt - contains the unique list of xref sources from the xrefs above in a format suitable for loading into the existing external_db table

Doesn't do dependent xrefs yet.

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