JPBFT-1: repository layer

[SvyatoslavK]

Add repository layer to the project.

Acceptance criteria:

- Flyway added
- Schema for database added
- Spring Data added
- Entities added
- H2 database for testing purposes added
- integration tests for with h2 added

[VitalyOrfenov]

Hint for second point:
Use @entity annotation to create DB table and specify it's properties
Create -Repository class, extended from CRUDRepository(/PagingAndSortingRepository/JPARepository if needed)

[VitalyOrfenov]

Don't forget about unit-tests