SafeDeleteFile should return true if file doesn't exists

TryMove fails when deleteExistingTo is true and there is no existing file.
SafeDeleteFile should return true if the file doesn't exists.
WindowsStore MvxPhoneCallTask doesn't compile

My bad, I messed up with the argument name in my last pull request
Merge pull request #690 from guillaume-fr/patch-4

SafeDeleteFile should return true if file doesn't exists
Merge pull request #692 from guillaume-fr/patch-5

WindowsStore MvxPhoneCallTask doesn't compile