Pub crates types are used

[therustmonk]

With current version we have to duplicate dependencies r2d2_postgres uses.

extern crate r2d2;
extern crate r2d2_postgres;
extern crate postgres;

and Cargo.toml:

[dependencies]
r2d2 = ">= 0.6, < 0.8"
postgres = "0.11"
r2d2_postgres = "0.10.1"

Sometimes it gives an error:

    = note: expected type `&[&postgres::types::ToSql]`
    = note:    found type `&[&postgres::types::ToSql]`
note: Perhaps two different versions of crate `postgres` are being used?

This PR makes possible to use one dependency only:

extern crate r2d2_postgres;
use r2d2_postgres::r2d2;
use r2d2_postgres::postgres;

[dependencies]
r2d2_postgres = "0.10.1"