[FIX] Package duplication with setuptools

Fix #185
Fix addons not found error with odoo.py after setup.py
Include addons in virtualenv site-packages egg directory
```
env/lib/python2.7/site-packages/openerp-8.0alpha1-py2.7.egg/addons
```
Remove duplicate openerp directory at root of virtualenv
```
env/openerp
```
Would also create opportunity to remove the gen_manifest function ?