7.0: add budget_invoice and budget_crm

Hi, this adds:

- `budget_invoice` to generate draft invoices from budgets
- `budget_crm` to to generate budget lines from opportunities
- some updates to the `budget` module to handle the `active` version of a budget

For details, see the `__openerp__.py` files in the `budget`, `budget_invoice` and `budget_crm` modules.

You can also check the yaml test cases to figure out how that is meant to be used.

Thanks!