Multi attachment

Allows templates to generate multiple attachments from different models using openerp domains. Reports can be from different models from the template one. The only thing you have to do is write a valid domain expression looking for required ids.

As an example if we have a model called ```model.one``` and you can associate its name inside ```origin``` field from invoices, you can send invoice by email using a template attached to ```account.invoice```, generating a report associated to ```model.one``` using a domain like ```[('name', '=', '${object.origin}')]``` 

This way the email can have two attachments. The invoice report and the ```model.one``` report.