Add Postgres serial field to contrib.postgres

[Johannes Maron]

Since we have the beautiful contrib.postgres package now, we can add a couple more Postgres specific fields, that are not supported by MySQL but handy to all the Postgres fan boys.

That being said, I guess it's a good idea to start with Serial.

The PostgreSQL documentation: ​https://www.postgresql.org/docs/9.1/static/datatype-numeric.html describes the field as follows:

The data types serial and bigserial are not true types, but merely a notational convenience for creating unique identifier columns (similar to the AUTO_INCREMENT property supported by some other databases).

Use Cases:

1. Anywhere an automatic incrementing value is required, but a primary key is often (mis)used.
2. Providing additional (user controlled) auto-incrementing values.

The primary benefit is that is isolates the primary key for the sole use of maintaining referential integrity.