[WIP] add support for xUnit 2 test runner

Fixes #609 

 - [x] resolve xUnit executables based on 32/64bit OS
 - [x] implement resolver which supports multiple files
 - [ ] make my F# code suck less
 - [ ] test this against a real environment
As this is returning the file name when it cannot be found, I think this is always going to default to the first entry. But you get the idea anyway...