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
A list is a seq. Do you need to convert here?

If so you could do: seq { yield "xunit.console.exe"; yield "xunit.console.clr4.exe" }