Create module to get medline archives from NLM

ftp_update_job will download all files from the NLM server that
were not previously downloaded, using a sqlite database as an
information store.

Currently using the stock Python ftplib. Note that on AWS, PASV
mode will cause the least headaches in firewall configuration; this
is the default in ftplib.

Next, need to create scripts using the functions defined in ftp_update_job.