[ADD] 7.0 'pos_second_header' new module to have an extra header in POS

Add 'pos_second_header' in 7.0 serie.

This PR has been previously approved on launchpad:
* @ovnicraft :+1: 2014-07-30
* @pedrobaeza :+1:  2014-09-23

Ref: https://code.launchpad.net/~sylvain-legal/openerp-pos/7.0_pos_second_header/+merge/228820