give multiple arbitrary addons path to server

I've written this blueprint in relation with bug #507973 where a trashy patch is attached.
https://bugs.launchpad.net/openobject-server/trunk/+bug/507973

The idea is to have the ability to pass more than one addons path when launching server. The same affect server configuration file.

ex : ./openerp-server.py --addons-path /home/mysur/src/trunk/addons , /home/mysur/src/trunk/extra-addons , /home/mysur/src/dev/my-modules , /home/myusr/work/project/customer1/mycustomer1_modules

Actually when config['addons_path'] is used in addons it's to physically
access to server/bin/addons folder to get ressources, ie access a file
within a folder, so it mlight be more logical to python methods or openerp-server methods like get_module_path.

When config['addons_path'] is used in core parts, it's for appending to
python path with sys.path.insert, this one support dict.

We also have in bin/addons/init.py some functions that deal with modules
management but doesn't use config['addons_path'] really...

For now i don't really know what's the best way but basically we need :

- a method for adding extra addons path on startup (command line) or
server config file. This should make useless the usage of symbolic links
and in develppement process a more efficient way for switching between
different setup environnement or configuration for testing extra-addons
or community modules for example.

What's important is to have a way to add arbitrary module folder or
parent modules folder.

Maybe a new command line arg like --extra-addons-path can be a pretty
way for doing it.

Any thought are welcome.