[FEATURE] Add read write connection support

[fureszpeter]

Changes proposed in this pull request:

• EntityManagerFactory::create avoid re-parse the config
• Can handle MasterSlaveConnection (http://www.doctrine-project.org/api/dbal/2.0/class-Doctrine.DBAL.Connections.MasterSlaveConnection.html)

Important Notes

Since Laravel support only one slave (read-only) connection, not completely compatible with Laravel database.php

Differences:

_Doctrine version:_

            'read' => [
                [ //<==Please note, we need to add multiple arrays here!
                    'host' => env('DB_HOST_R', 'localhost'),
                    'database' => env('DB_DATABASE_R', ''),
                    'username' => env('DB_USERNAME_R', ''),
                    'password' => env('DB_PASSWORD_R', ''),
                ],
            ],

_Laravel version:_

            'read' => [
                    'host' => env('DB_HOST_R', 'localhost'),
                    'database' => env('DB_DATABASE_R', ''),
                    'username' => env('DB_USERNAME_R', ''),
                    'password' => env('DB_PASSWORD_R', ''),
            ],

[patrickbrouwers]

Need some time to properly test this, so it doesn't break any existing functionality.

[guiwoda]

@fureszpeter Could you separate your EM config change from your read-write change?
I think they could be merged individually.

[patrickbrouwers]

Replaced by #214