User Aggregate reference inside TaskManager BC

[benatespina]

After merge #131 PR we have modified the vision of User aggregate inside TaskManager core BC.
At first, we only have a UserId VO, but to make possible the proper sync of Users with organization management, we have added the User AggregateRoot and its repository.

This is not the Vaughn Vernon approach. But we haven't found any other solution to validate the existence of Users inside TaskManager core BC.

Some links about problem:

• https://blog.codecentric.de/en/2015/04/bounded-contexts-and-data-duplication-in-action-adding-a-shop-system-to-the-movie-database/
• http://stackoverflow.com/questions/18761001/ddd-how-to-design-associations-between-different-bounded-contexts