Allow slick 2.0 driver override in config.

I wanted to use slick-pg with the slick20 branch and found out that issue #25 is not solved in that branch.
So, here is a port of pull request #110 to slick 2.0.
In the mean time, I added a way to load the driver class from the play application's class loader. (Useful if the custom driver is in the play's re-loadable class loader).