Add Makefile to download taxonomic reference data

[erictleung]

• Makefile to download reference data for Silva, RDP, GreenGenes, UNITE (fungal)
• Add test in verification for checking at least one taxonomic reference database included before executing workflow
• Check URL links are still valid (e.g https://stackoverflow.com/questions/25135347/how-to-check-status-of-urls-from-text-file-using-bash-shell-script)
• Use MD5 checking for databases