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.

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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, now I see what my problem is.

Although you've changed the *types*, the code using `Success` and `Failure` didn't change, so GitHub's diff view doesn't show me the usage. It looked to me as though you'd just added that active pattern, but because you hadn't *added* any code that uses the active pattern, I didn't see the *existing* usage of `Success` and `Failure` in the diff view...

Anyway, since we're pulling in FSharpx.Core already, why not use the active pattern it defines?