Switched NSDataDetector from "nonatomic" to "atomic"

Fixes crash in issue #436.
Solution suggested by @ChrisLoer: "That sounds similar to issue #312 , and although that issue is closed, I think there's still a potential problem with access to the nonatomic dataDetector variable from multiple threads. I use a local fork with that variable declared atomic, and for what it's worth I haven't seen any more setText crashes since introducing that change."