[ADD] purchase_budget_limit

New module to block purchase orders when the budget is at risk of being exhausted

Supersedes this merge proposal from Launchpad: https://code.launchpad.net/~numerigraphe-team/purchase-wkfl/7.0-add-purchase_budget_limit/+merge/219829

"Here is a new module to block purchase orders when the budget is at risk of being exhausted.

Only validated budgets are checked.
Also, only invoiced purchases are counted (because only invoices are counted in the budget). I'll soon be publishing an additional module to count draft purchases in the budgets too.

Unfortunately, no automatic test bundled because the account_budget does not provide any either."
I'm not sure "today" is the best date from which to compare the budget lines here. I think the date_order field on PurchaseOrder would more accurate.