Corrected mistakes in the tutorial.

Ordered the entities to be written by the reader, in a more
logical manner.

Corrected code listings of JPA entities. Corrected the Data
Persistence chapter to include missing entities.

Corrected the code listing for the BookingServiceTest in the
Business Logic chapter. Added a tip linking the Arquillian guides.
Additional corrections. Mostly in building the frontend
Corrections in various chapters.

Corrected code snippets. And included additional instructions where absent.