Added error level setting to XUnitHelper, similar to NUnit error level.

As per @forki's comment [here](https://github.com/fsharp/FAKE/pull/263#issuecomment-30671364), added an error level setting to `XUnitHelper`.

However, there is a problem, as the case names clash with the ones of `NUnitErrorLevel` ([see](https://github.com/blacktaxi/FAKE/blob/develop/src/app/FakeLib/NUnit/Common.fs#L10)). So when configuring xUnit runner you have to explicitly qualify the type for `XUnitErrorLevel`, which might be confusing to novice F#-ers.

Possible solutions:
-   Define a common error level type and use it in both test runner helpers. Possibly a better solution.
-   Rename the cases of one of the types so they don't clash. This could work, but might be confusing as well, as the cases are ought to represent the same thing but for different runners, so they probably should have the same name.
-   Variation of the previous one: add a prefix to each case of both of the `NUnitErrorLevel` and `XUnitErrorLevel`. I think this is a bit less clumsy.
-   Change the `XUnitHelper`'s `Error : XUnitErrorLevel` to `DontFailBuild : bool`.

What do you think?