Enum

Adds support for enum field types in migration generation and scaffold generation. For example:

`artisan generate:scaffold dog --fields="name:string, breed:enum[array('lab'|'poodle' | \"pit bull\")]"`

Will generate this line in a migration:

`$table->enum('breed', array('lab','poodle' , "pit bull"));`

When the forms are generated, they will contain a dropdown with the available options. 

![screenshot from 2013-11-26 18 35 59](https://f.cloud.github.com/assets/409617/1623830/f6ee0a08-56b8-11e3-9407-e583cdccde69.png)