Add subpackage support

As project grows keeping all entities/repositories/etc at the same folder level makes it hard to focus on the functional groups of the project. Therefore I added a second parameter to the entity generator that allows us to specify sub-packages:

    $ yo jhipster:entity EntityA
    The entity EntityA is being created.
       create src\main\java\com\mycompany\myapp\domain\EntityA .java
       create src\main\java\com\mycompany\myapp\repository\EntityARepository.java
      ...
     $ yo jhipster:entity EntityB packageb
    The entity package.b.EntityB is being created.
      create src\main\java\com\mycompany\myapp\domain\packageb\EntityB.java
      create src\main\java\com\mycompany\myapp\repository\packageb\EntityBRepository.java

Also moved the User, PersistentToken and Authority to the "security" sub-package