Add script fetch-emacsmirror-repos.pl

This script fetches all the repositories on the "emacsmirror" user
account and stores the new ones in a PostgreSQL database.

The database that will contain the repositories is specified in a
"postgres://" URL on the environement variable DATABASE_URL.
Add packages DBI and DBD::Pg to Makefile.PL

This will cause the packages DBI and DBD::Pg to be installed when
Heroku compiles this web application.

These packages are needed due to the new script
"bin/fetch-emacsmirror-repos.pl"