PostgreSQL with the peer authentication method.

Added information how to get owncloud to use PostgreSQL which is
configured to use the peer authentication method. This is the default
setting in Ubuntu 14.04 (and probably other distros).

Information was adopted from
https://gitlab.com/gitlab-org/gitlab-ce/blob/7-1-stable/doc/install/installation.md#4-database
and tested this with ownCloud 7.0.1 (stable).
More information on how the parameters must be chosen.
using .. note:: instead of IMPORTANT:
Merge pull request #482 from owncloud/psql-peer-auth

Psql peer auth