Async user mapper

Adds Tests For [PR 1643](https://github.com/NancyFx/Nancy/pull/1643).  Also "Nancy-ifies" the code to keep it stylistically similar to the rest of Nancy. (Only one small change). It fixes a bug in the otherwise excellent implementation provided by @ExUltima (the UserMapper was still required even if an Async one existed).
then 
```
if (!((this.UserMapper != null) ^ (this.AsyncUserMapper != null)))
```
seems more straightforward to me.

Having said that, I think this whole discussion shows that the expression should be extracted to a method named ``IsUserMapperConfigurationValid`` or something like that.