Add -j option to db:restore to use multiple jobs to restore databases

[petitJAM]

Like with #51, there's an option to speed up DB restores. Passing -j to pg_restore will cause it to use more than 1 process/thread for restoring a database. This is useful for large database dumps that take many minutes to restore.

From pg_restore --help:
-j, --jobs=NUM use this many parallel jobs to restore

This option could also potentially be automatically set to some number. This SO answer https://stackoverflow.com/a/6481016 has a way to check for the number of CPUs on the system. I don't know that we would want to use the max number since that might wreck the computer it's running on, but maybe half or something?