Add support for percentiles aggregation

[im-denisenko]

Percentiles aggregation was available since ES 1.1, but still marked as experimental.
But like scripted metrics in #780, I need it and thought maybe you want to merge :)

[im-denisenko]

Btw, anyone who has older version of elastica and wants to use not implemented aggregation (just like me today), they can use following workaround. It's less hardcore than raw array query:

$query = new Elastica\Query();
$query->setFilter(/* blah blah */);
$query->setQuery(/* blah blah */);
$query->setParam('aggs', [
    'price_percentiles' => [
        'percentiles' => [
            'field' => 'price',
        ],
    ],
]);

[coveralls]

Coverage decreased (-0.2%) to 84.13% when pulling c69a1f2 on im-denisenko:feature/percentiles-aggregation into bfa9363 on ruflin:master.

[coveralls]

Coverage increased (+0.04%) to 84.37% when pulling c69a1f2 on im-denisenko:feature/percentiles-aggregation into bfa9363 on ruflin:master.

[ruflin]

Merged. It would probably make sense to have your comment above as documentation on Elastica.io.

[im-denisenko]

With my english level, some text longer than PR message, always looks terrible.
I can have a try in near future, just don't wait for this soon.

[ruflin]

@im-denisenko No worries about your english. We all make some mistakes.