Added Prop.ofTestable.
Added Prop.ofTestable.

forki/FsCheck,fscheck/FsCheck,sideeffffect/FsCheck,cloudRoutine/FsCheck,yukitos/FsCheck,fscheck/FsCheck,vasily-kirichenko/FsCheck,forki/FsCheck,fsharp/FsCheck,dungpa/FsCheck,yukitos/FsCheck,ploeh/FsCheck,paulyoung/FsCheck,cloudRoutine/FsCheck,yukitos/FsCheck,jhamm/FsCheck,kos59125/FsCheck,ErikSchierboom/FsCheck,paulyoung/FsCheck