Add "exists" to GeneratorDrivenPropertyCheckrs

Two things to note about this, based on ScalaCheck's Prop.exists method implementation.

  1. Like ScalaCheck, only single-arity functions are supported (e.g. exists(...) { a: String => assert(a.length == 1)  }, but not { (a: String, b: String) => ...}
  2. MinSuccessful and MaxDiscardedFactor config settings appear to *not* be supported by Prop.exists.   That is, checking Prop.exists with different maxDiscardedRatio values does not appear to affect the result.   I've disabled directly passing minSuccessful, etc property values, but confusion could arise because the user can still set the minSuccessful, etc properties on a PropertyCheckConfiguration instance.