Support PostgreSQL

[firstred]

This PR adds support for PostgreSQL by adjusting the migrations.

[mamazu]

Hey first of all thank you for adding migrations to support postgres. I have a couple of questions about this:

1. How did you "generate" those migrations? Is there a flag that you used to because then we might want to document that as well so that we can create them correctly in the future. Or did you write them all by yourself?
2. Correct me if I am wrong but I have read somewhere that the best approach to migrations would be to generate them on the project you are using the plugin in. So I though we might as well remove the migrations entirely and write just run doctrine:migration:diff into the docs.

With all this to say I am inclined to merge this since the plugin is pretty much "done".