Run tests on both k10 and net45

Third time's a charm...

@davidfowl @loudej Would you mind glancing through this? I modified the k task to take an optional runtime parameter. If specified, it basically does a k list and picks one with the specified runtime and the same version and architecture as the active one.
I would love to, but I have no time...

Taking a look at _k.shade it seems like it might be simplest to chain in the kvm call like "/C kvm use default -svrc50 && k ${command}" that's the only way it could work, because kvm.cmd will only affect the current process - when it returns the build process's PATH won't be modified at all. 

So yeah, I'd suggest adding a `default kvmUse=""` to the _k.shade, and prefixing the call as `"/C kvm use ${kvmUse} && k ${command}"` when it's provided... then the caller can say kvmUse="default -svrc50" or whatever