Suggestion: optional columns in createTable

To be used like this:
```
$builder->createTable('table1',array(
    'field1'=>array(Schema::DT_INT,TRUE),
    'field2'=>array(Schema::DT_INT,FALSE),
    'field3'=>array(Schema::DT_INT,FALSE,123),
));
```

It should help for performance + it allows not nullable fields without default values (cf. issue #10)