[RavenHandler] Add support for contexts

[rvalitov]

Originates from #1179, and problem discussion is available at Sentry.

Sentry provides contexts to add extra data to the log. But it's not supported in current release of RavenHandler. As a result, in Sentry we see empty info like this:

This PR fixes this, so that user can pass contexts field in the record, as a result we will see something like this:

Users can pass information to Sentry like this:

$logger = new Monolog\Logger('YOUR_LOG_NAME');
$client = new Raven_Client('YOUR_SECRET_KEY');
$stream = new RavenHandler($client);
$log->addNotice('Some information', [
    'contexts' => [
        'os' => [
            'name' => 'Windows',
            'version' => '10'
        ]
    ]
]);

It's also possible to pass the contexts using processors. Example:

$log->pushProcessor(function ($record) {
    $record['contexts'] = [
        'os' => [
            'name' => 'Windows',
            'version' => '10'
        ]
    ];
    return $record;
});

[mleczakm]

Useful contribution (I could remove some part of my bundle ;) if it only were merged, thanks a lot) but - missing tests.

[rvalitov]

I'll try to add the tests later...

[Seldaek]

I fixed this in a simpler way, that only allows contexts to be passed either in the context array or in the extra (which is where processors should put their data, not as a random key on the $record array).