Add support for PostgreSQL

[aaronbrethorst]

Let's make it possible to use the PostgreSQL database in addition to MySQL. MySQL should remain the default.

Here's what I know should change:

1. Allow the specification of the JDBC driver class in data-sources.xml
2. Add the PostgreSQL JDBC driver to the app server docker image (https://jdbc.postgresql.org)
3. Update the README with information on using PG instead of MySQL

I'm sure I'm forgetting something here, so use your best judgment on getting this built out!

[CesarAndrade007]

I'd like to take this on @aaronbrethorst!

[aaronbrethorst]

Any chance you might have the bandwidth to tackle this, @Altonhe?

[Altonhe]

@aaronbrethorst no problem

[Altonhe]

Blocked by OneBusAway/onebusaway-application-modules#352