Add MySQL enum support

This PR allows a user to create enum columns using MySQL

Usage is as follows:

```
// Normal usage - pass a PHP array in for the 'limit' option
$this->addColumn('column_name', 'enum', array('limit' => array('value1', 'value2'))

// or create a blank enum (not sure why you would do this, but it is supported since MySQL supports it)
$this->addColumn('column_name', 'enum', array('limit' => array())

// creating an enum column and not specifying a limit will throw a PDOException
$this->addColumn('column_name', 'enum')
```