Add Postgres implementation

[localhuman]

One of the goals of this project is to allow users to choose their backend implementation. I think the first step in that direction would be to add an alternative implementation to LevelDB which might be a bit more user friendly for people to work with. My preference would be postgres.

[metachris]

Postgres would be very nice indeed! It's an amazing database.

What about using SQLAlchemy? It would then be compatible with SQLite, Postgresql, MySQL, Oracle, MS-SQL, Firebird, Sybase and others.

[ixje]

Closing due to focus on NEO3