add init from key

[lachlankrautz]

Added initFromKey($key) to get an enum instance from the key.

Mostly useful to convert the db value stored into the Enum on a model.

class Animal extends Enum
{
    const CAT = 'kitty';
    const DOG = 'dog';
    const HORSE = 'horse';
    const PIGEON = 'skyrat';
}

// Init from key in variable (probably loaded from db, or submitted by user)
$kitty = 'kitty'; 
$cat = Animal::initFromKey($kitty);