Re-export lifted Test.Hspec expectations

Addresses https://github.com/hspec/hspec-wai/issues/22
My reasoning was more along the lines of "is a wep app that responds with 500 well-behaved?"?  I would tend to say "no", for me 500s are for cases that happen on production that the programmer did not think about in advance.

On the other hand, you may indeed want to test what happens if your DB server is not available.  Most applications would probably fail with an exception which would be turned into a 500 by the application server.  But then again, if you already write a test for it, you may want to go the extra mile and return a more appropriate status code (say 503) in that case...

But there is something else, with the type:

```haskell
shouldThrow :: Exception e => IO a -> Hspec.Selector e -> WaiExpectation
```

We can not use this combinator in the `WaiSession`-monad, as it's first argument is of type `IO a`.

I would tend to omit it for now.  We can add it later if there is demand for it (so if you need it frequently, please bug again ;).