Add Enum.Parse<T>() extension

Parsing a string value to an Enum value is quite cumbersome. For example:

```
order.State = (OrderStates)Enum.Parse(typeof(OrderStates), "Completed");
```

I'm thinking something simple, like a strong-typed generic helper like the following idea makes sense: 

```
order.State = Enum.Parse<OrderStates>("Completed");
```
