Remove unused imports

Initial step for #14
Add regex to validate email address

Hibernate's @Email will accept address such as 'user@domain'. This however is not a valid email address. Add another regex which supports standard email address format.

Fixes #14
Add ControllerAdvice to massage errors being sent to client

Default error message that being sent to client is difficult to read. It contains to much unnecessary information. Remove those information and only return relevant information to avoid difficulty for clients to understand.

Fixes #14
Add tests to validate data being sent by client

Add tests to ensure that all required information is provided. Also validate that email format is correct.

Fixed #14