Add Customer, Product, RegionContext, Supplier and Tax Persistence services. Change Buisness to Business
Change PTBuisness to PTBusiness
Add update method
Update README.md
Merge develop-Persistence
Change PTInvoice reference in CreditNote Builder to UID instead of direct reference
Start concurrency tests. Add constraints to GenericInvoice
Start test fix
Merge branch 'concurrency-fix' of github.com:premium-minds/billy into concurrency-fix

Conflicts:
	billy-portugal/src/test/java/com/premiumminds/billy/portugal/test/services/documents/TestConcurrentIssuing.java
	billy-portugal/src/test/java/com/premiumminds/billy/portugal/test/services/documents/TestDocumentIssuingService.java
	billy-portugal/src/test/java/com/premiumminds/billy/portugal/test/services/jpa/TestJPAPTBusiness.java
	billy-portugal/src/test/java/com/premiumminds/billy/portugal/test/util/ConcurrentTestUtil.java
Fix headers