Treat instances of DateTimeInterface as a value type throughout the API

[TimeToogo]

For instance:

$traversable = Traversable::from([
    ['date' => new \DateTime('1/1/2000'), 'message' => '...'],
    ['date' => new \DateTime('1/1/2000'), 'message' => '...'],
    ['date' => new \DateTime('2/1/2000'), 'message' => '...'],
]);

$results = $traversable
    ->groupBy(function ($row) { return $row['date']; })
    ->select(function (ITraversable $group, \DateTime $key) {
        return $key->format('d/m/Y') . ':' . $group->count(); 
    })
    ->asArray();

Due to strict comparisons on the group by this would yield some rather unexpected results:

['01/01/2000:1', '01/01/2000:1', '02/01/2000:1']

When what is actually wanted is:

['01/01/2000:2', '02/01/2000:1']

This PR changes the Identity::hash behaviour such that DateTimes are treated as a value type based on their timestamps (and class name) which would solve this problem.

[coveralls]

Coverage increased (+0.0%) to 91.27% when pulling 562d0f1 on datetimes-as-value-type into 00514d8 on master.

[coveralls]

Coverage decreased (-0.05%) to 91.24% when pulling e65ae5a on datetimes-as-value-type into e528d48 on master.

[coveralls]

Coverage increased (+0.0%) to 91.29% when pulling e65ae5a on datetimes-as-value-type into e528d48 on master.

[Ocramius]

Just a note (and I know I'm against PHP conventions here), but I like how other languages distinguish :: and # for static/instance methods ;-)

[coveralls]

Coverage decreased (-1.5%) to 90.903% when pulling 2347367 on datetimes-as-value-type into ad76515 on master.