Always allow proxies on ToOne associations

[goetas]

Hi!
This patch combined with doctrine/common#338 will allow to have proxies for long chains of ToOne associations:

class User{
  private $id; // @id
  private $city; // @id
  private $name;
}
class City{
  private $id; // @id
  private $region; // @id
  private $name;
}
class Region{
  private $id; // @id
  private $country; // @id
  private $name;
}
class Country{
  private $id; // @id
  private $name;
}

$user = $repo->find(1);

// the current implementation must load: city and region
$user->getCity()->getRegion()->getCountry();
// the current implementation must load: city, region
$user->getCity()->getRegion()->getCountry()->getId();

//with this patch
// here no queries will be executed
$user->getCity()->getRegion()->getCountry();
// here no queries will be executed
$user->getCity()->getRegion()->getCountry()->getId();

Of course this patch will require some unit test, but can be acceptable as idea?

[doctrinebot]

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[goetas]

@Ocramius I'm trying to explore a different approach, but this PR is "related"/"overlaps" to #1113 (always talking about foreign identifier entities...).

What is your opinion about #1113 ... is it acceptable?

[goetas]

At the moment I do not have time to work on it... if #1241 will be merged, the problem will be solved in better way