Added ability to pass Closure to union statement in query builder

Example:
```php
$users = DB::table('users')->whereNull('last_name')->unio(function($q)
{
	$q->from('users')->whereNull('first_name');
})->get();
```
some updates

shorten code, updated docblocks
Merge pull request #1165 from Ruby184/patch-2

Added ability to pass Closure to union statement in query builder