[REF] core: Remove psycogreen dependency

Purpose:
    The psycogreen module is unmaintained. Last source update was in
    2015 and last pypi package was in 2012. Odoo only use a small
    part of the psycogreen module that could be written directly in
    the odoo code. This commit reproduce the small function from
    psycogreen used in ODoo and remove all depencies from it.
    Also the copyright and license are honored and the documentation
    is updated accordingly.

    pypi pckage: https://pypi.python.org/pypi/psycogreen
    bitbucket repos: https://bitbucket.org/dvarrazzo/psycogreen/