Added unit test for InvoiceManager
Merge branch 'master' of https://github.com/CSBill/CSBill

* 'master' of https://github.com/CSBill/CSBill:
  Fixed callback error on grid filters
  Updated entities to use traits for timestampable and softdeleteable
  Updated minimum required PHP version to 5.4.0
Removed invalid SQL in migrations, and enable debugging in InvoiceManagerTest kernel