Support Collection->select and Collection->column


Collection->select() allows for selecting only a subset of columns to be fetched. This keeps the memory usage minimal in situations where you really only need certain columns. At present missing columns will return NULL and saving partial objects behavior is undefined.

```php

$llamas = Animal::find()->select(array('llamaid', 'type'));
```

Collection->column() makes it easy to get a simple array of a particular column:

```php

// returns an iterator for iterating over an array of integer id's
$ids = Animal::find()->column('llamaid');

// find unique types, returns an array
$types = Animal::find()->column('type')->unique();

```