[person] Feature: add a person's center history

The association between Person and Center is now stored in a dedicated
Entity: `PersonCenterHistory`, which have a date interval (start date
and endDate). The SQL counterpart is a table, with a constraint which
ensure that no person might be associated with two center at the same time.

For ease, a view is created to get the current center associated with
the person.

The dedicated migration creates also:

* indexes for a rapid search for person at current date;
* and populate the table from current data, setting the startdate to the
  person's creation date and time if any, `NOW()` unless.

The `Person` entity is also updated to use the information from the
PersonCenterHistory classes, but this commit does not yet delete the
`Center` column.

src/Bundle/ChillPersonBundle/Repository/Person/PersonCenterHistoryInterface.php

@@ -0,0 +1,10 @@
+<?php
+
+namespace Chill\PersonBundle\Repository\Person;
+
+use Doctrine\Persistence\ObjectRepository;
+
+interface PersonCenterHistoryInterface extends ObjectRepository
+{
+
+}