[ADD] added rating, rating_project and rating_project_issue module

 - Task : https://www.odoo.com/web#id=8060&view_type=form&model=project.task
 - Pad : https://pad.odoo.com/p/r.2BmKqyvcwKbiXPRI

[IMP] Added option to install rating module under Project

[IMP] added template to display when customer rates the task/issue

[IMP] Improved template and code

[IMP] : rating , rating_project and rating_project_issue as per new api

[IMP]: rating template

[FIX] : test case failed

[IMP] : code
Sorry, I just read a comment of BST on the pad. It's even better to use a larger scale to adapt number (1,2,3,...) to grade (great, okay, and bad). With only 5 number, it is not gonna be easy, so the field must looks like :
fields.Selection([(-1, 'No value'),(0, '0'),(1, '1'), (2, '2'), (3, '3'), (4, '4'), (5, '5'), (6, '6'), (7, '7'), (8, '8'), (9, '9'), (10, '10')], string="Grade", type='float', group_operator="avg", default=-1)
Since we set type=float, the 0/False/NULL problem is fixed ; 0 appears as 0 in the database. 
This scale is good to use smiley and star rating :smile: