[PROPOSAL] Add ability to automatically resolve DB::connection('mysql::write') to use writable database connection as well as DB::connection('mysql::read') instead of adding duplicate connection config.

| Q             | A
| ------------- | ---
| Bug fix?      | no
| New feature?  | yes
| BC breaks?    | no
| Deprecations? | no
| Tests pass?   | yes
| Fixed tickets | N/A
| License       | MIT
| Doc PR        | N/A

## Current Behaviour

You are required to create separate database configuration if you need to force using either "read" or "write" mode as suggested in <http://laravel.io/forum/05-25-2014-read-write-mysql-connection-forcing-a-select-to-use-write-connection>.

```php
'mysql' => array(
    'driver'    => 'mysql',
    'read' => array(
        'host' => 'read-host',
        'database'  => 'read-database',
        'username'  => 'read-username',
        'password'  => 'read-password',            
    ),
    'write' => array(
        'host' => 'write-host',
        'database'  => 'write-database',
        'username'  => 'write-username',
        'password'  => 'write-password',               
    ),
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
),
'mysql-write' => array(
    'driver' => 'mysql',
    'host' => 'write-host',
    'database'  => 'write-database',
    'username'  => 'write-username',
    'password'  => 'write-password',               
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
),
```

```php
DB::connection('mysql-write')->select(...);
User::on('mysql-write')->find(1);
```

## Proposed Behaviour


```php
'mysql' => array(
    'driver'    => 'mysql',
    'read' => array(
        'host' => 'read-host',
        'database'  => 'read-database',
        'username'  => 'read-username',
        'password'  => 'read-password',            
    ),
    'write' => array(
        'host' => 'write-host',
        'database'  => 'write-database',
        'username'  => 'write-username',
        'password'  => 'write-password',               
    ),
    'charset'   => 'utf8',
    'collation' => 'utf8_unicode_ci',
    'prefix'    => '',
),
```

```php
DB::connection('mysql::write')->table('user')->get();
User::on('mysql::write')->get();
```

Both above code should force using "write" pdo connection.

## Limitation

* Depending on the use case, this could actually increase your database connection count as separate PDO connection is created for "mysql" and "mysql::write" connection name.

Signed-off-by: crynobone <crynobone@gmail.com>