Merge: Wrap postgresql #1810

Merge: Wrap postgresql #1810

Started working on a wrapper for PostgresSQL as per issue: https://github.com/nitlang/nit/issues/1810

So far this wrapper provides basic functionality allowing:

* a connection to be made
* a statement to be executed
* creation of prepared statements
* executions of prepared statements
* connection status functions

according to the [PostgresSQL libpq interface](http://www.postgresql.org/docs/9.5/interactive/libpq.html). It's about equivalent to the `native_sqlite3.nit` class.

Just wondering how much more functionality we want to add and looking for some review on the usage of the FFI and style.

@xymus @privat

Pull-Request: #2051
Reviewed-by: Alexis Laferrière <alexis.laf@xymus.net>
Reviewed-by: Jean Privat <jean@pryen.org>