Exchange: add modal validations working.
Exchange receipts working now for events and resources, with inventory rule.  Removed edit button.
Revert change to models.EconomicEvent.save to merge without conflict, will test before pushing merged version.
Merge branch 'master' of https://github.com/valnet/valuenetwork.git

Conflicts:
	valuenetwork/valueaccounting/models.py - resolved by taking master (save of EconomicEvent)
Exchange: minor UI thing.
Merge branch 'master' of https://github.com/valnet/valuenetwork.git