Consider adding a specific table for invoices

[mircobabini]

After I pulled the request about filtering the generation of invoices (in my case must be generated only for orders above 100$) it's necessary to know which invoices should be generated and which not.

Then we need a table for this purpose. This way I can:

• offload pdf files somewhere else (instead of bloating the website space)
• re-generate the invoices simply by deleting the files and re-run the generation script (at the moment regenerates all invoices, doesn't fit my case)