Where features (IN, BETWEEN) and custom operators ( >=, <=, etc)

Custom Operators:
```
$db->where( 'id', array( '>=' => 50 ) );
// Gives: SELECT * FROM posts WHERE id >= ?
```

BETWEEN: 
```
$db->where( 'id', array( 'between' => array(4, 20) ) );
// Gives: SELECT * FROM posts WHERE id BETWEEN ? AND ?
```

IN: 
```
$db->where( 'id', array( 'in' => array(1, 5, 27, -1, 'd') ) );
// Gives: SELECT * FROM posts WHERE id IN ( ?, ?, ?, ?, ? )
```