Add enum type creation to DSL

[mariogarcia]

Example:

DSL.enum('Countries') {
    description 'EU countries'

    value 'SPAIN', 'es'
    value 'ITALY', 'it'
}

It seems that if you declare a method like:

def 'enum'(String name, Closure dsl) {
   // implementation
}

Then you can call enum('Countries') { }