Removed support for OpenERP 5.0
Major removal of support for OpenERP < 8

Notably:

- no more dance around PIL/Pillow, Odoo has settled to official Pillow dep
- no more openerp-command and its version-dependent behaviour. Chances are
  that oe won't come back
- no more GTK and Web clients

The infrastructure for version-dependent adaptation has been mostly kept as is
Minor refactors will probably be in order