Add support for PostgreSQL and other databases

[carlosonunez]

While I was completing this challenge, I made the mistake of thinking before
looking at this codebase that I could use any database.

I chose PostgreSQL out of familiarity. I chose poorly.

I wasn't sure whether modifying my infrastructure code to use Mongo would be
faster than just shimming in Postgres support into this codebase, so I went wit
the latter because it was more fun.

This pull request adds support for persisting Tasky to-dos into PostgreSQL. It
also decouples the persistence layer through a simple interface so that others
can add support for more databases, like SQLite or MySQL.

I hope this helps!

[jeffthorne]

Thanks Carlos 👍 Will Review.