Add Arrays::extract()

This method will take an array such as 
```php
$resultSet = [                                                                                                           
    ['key' => '108273824', 'value' => 'http://www.example.com'],                                              
    ['key' => '166799518', 'value' => 'N'],                                                                              
    ['key' => '872314986', 'value' => '9999.99'],                                                                        
    ['key' => '945587452', 'value' => ''],                                                                               
    ['key' => '773529024', 'value' => 'Y'],                                                                              
    ['key' => '872203238', 'value' => 'Y'],                                                                              
    ['key' => '874939330', 'value' => '5551234567'],                                                                     
]; 
```
and produce an array such as
```php
array(8) {
  [108273824] =>
  string(33) "http://www.example.com/"
  [166799518] =>
  string(1) "N"
  [872314986] =>
  string(7) "9999.99"
  [945587452] =>
  string(0) ""
  [773529024] =>
  string(1) "Y"
  [872203238] =>
  string(1) "Y"
  [874939330] =>
  string(10) "5551234567"
}
```

Need a name for the method. Take a look at the `FIXME` tags.