Fix blank page when returning receivings

Conflicts:

	application/libraries/Receiving_lib.php
Add invoice number duplication check in receivings screen
Add invoice number duplication check in receivings detailed report
Merge branch 'feature/invoice_numbering' into merge/adewale

Conflicts:
	application/controllers/receivings.php
	application/language/en/receivings_lang.php
	application/libraries/Receiving_lib.php
	application/models/receiving.php