ShouldAllBe for IEnumerables

Hi!

First, I really like Shouldly (and so do my colleagues), so thanks a lot for it! 

Now, I found that while working with enumerables, I'd often want something that verifies that a certain condition holds for each element in an enumerable.

I discovered that `ShouldNotContain` with a negative condition actually does the trick, but it makes the test somewhat difficult to read due to the double negation.

For example,

```csharp
// verify that all words start with a "y"
var arr = new[] { "yes", "yay", "yellow" }

arr.ShouldNotContain(s => !s.StartsWith("y"));
```
Can now be written as

```csharp
arr.ShouldAllBe(s => s.StartsWith("y"));
```

Good idea? I included the obvious unit tests (mostly half-copied from existing tests).

ps. if you're looking for some help in maintaining Shouldly, I'd be happy to. I don't have any internet fame or credentials to speak of, though.