[ADD] 7.0 'pos_select_customer' to select customer in Front End POS

Add 'pos_select_customer' in 7.0 serie.

This PR has been previously approved on launchpad:
* @pedrobaeza :+1:  2014-09-11 
* @ovnicraft :+1: 2014-10-05 

(Obsolete Need Fixing by @lepistone 2014-08-04 )

Ref: https://code.launchpad.net/~sylvain-legal/openerp-pos/7.0_pos_select_customer/+merge/228826