Initial commit for implementing Enum

This is an attempt at implementing #1

Not a lot to go by in terms of the expected API for the Enum (or maybe I just didn't look hard enough in the existing specs?). Don't hesitate to let me know if I'm missing a few things.

BTW, I kind of like Jamis Buck's implementation too (http://weblog.jamisbuck.org/2005/8/7/enumerated-types-in-ruby) and I can see a way to make it work like so:

```ruby

class RoutingStatus < Enum
  NotRouted
  Misrouted
  Routed
end

RoutingStatus[:NotRouted]  #=> 0
RoutingStatus::Routed      #=> 2
RoutingStatus[1]           #=> RoutingStatus::MisRouted
```

Let me know what you think, especially re: your preferred API for this class.