Implement doctrine/collections on the ZoneMember class.

As discussed with Bojan, here is the implementation of Doctrine/Collections class in order to manipulate  collection of objects.

PHPUnit test: 

```
$ phpunit --verbose --strict
PHPUnit 4.3.5 by Sebastian Bergmann.

Configuration read from /home/vagrant/data/zone/phpunit.xml

...............................

Time: 934 ms, Memory: 8.25Mb

OK (31 tests, 75 assertions)
```