Add Enum Type

While it's possible to validate string enums using the regex type, a more comfortable solution should be available.

``` coffee
Alert = React.createClass
  propTypes:
    type: ReactProps.require(type: 'enum', options: ['info', 'warning', 'success', 'danger'])
```

(I initially noticed this after reading [](http://gcanti.github.io/2014/09/10/an-alternative-syntax-for-react-prop-types.html).)
