Allows choosing Postgresql

Does a couple things:

* Allows switching to postgresql from mysql (which is still kept as the default) with two small conf changes, and updates README.md to reflect this
* Updates deprecated rhc cartridge adding syntax (and shows old syntax just in case old versions of rhc still need it, though I wonder if the syntax is read on Redhat's servers anyway...)
* Minor formatting in the docs since I expanded them

The reason I chose to do this in the first place, by the way, is that I had migration problems with MySQL. Looking it up, it sounded like at least one of those problems were fixed in the latest HEAD of [master] in Sentry, but I didn't try since this was simpler. But, perhaps Mysql could be fixed by having requirements.txt point directly to that version. Since that's a risky proposition, I didn't go anywhere near that, I just wanted to give you a heads up on that in case you wanted to do something with it.