Used FSharpx.Core instead of Result<'TSuccess, 'TFailure> – refactored a nested pattern matching

Related to #6, and more specifically to this [comment](https://github.com/GreanTech/Qaiain/issues/6#issuecomment-51312114).

This Pull Request replaces the custom `Result<'TSuccess, 'TFailure>` type with the built-in `Choice<'T1, 'T2>` – the monadic type of the [`choose`](https://github.com/fsprojects/fsharpx/blob/9ad7ff3024b1cc90fd252520272421920c1f4017/src/FSharpx.Core/ComputationExpressions/Monad.fs#L806-809) Computation Expression.

Then it composes the handling of large messages in `handle` function using that Computation Expression, so we can avoid the nested pattern matching.

-----

As [FSharpx.Core](https://www.nuget.org/packages/FSharpx.Core/) has been included in source control, the entire diff is big – essentially there are 2 files to look at: *Program.fs* and *MailTests.fs*.

Ah, yes. FSharpx.Core defines this Active Pattern in its [source code](https://github.com/fsprojects/fsharpx/blob/cb9942ff7fb7f8a463070cc1c216b0170c399e28/src/FSharpx.Core/ComputationExpressions/Monad.fs#L772-775).

Instead of writing:

```f#
let actual =
    match "<bar" |> handle with
    | Choice2Of2 actual -> verified := expected = actual
    | Choice1Of2 _ -> verified := false
```

FSharpx.Core uses the Active Pattern and makes the Pattern Matching more readable:

```
let actual =
    match "<bar" |> handle with
    | Failure actual -> verified := expected = actual
    | Success _ -> verified := false
```

Since it's used in only two places, I didn't invoke the rule of three and just copied it. Thinking out loud, we could use the one defined in FSharpx.Core too.