Make non-executable prefixes configurable

```
The web services I consume use the following non-executable prefix:
throw 'allowIllegalResourceCall is false.';
This isn't compatible with Gson. We have to filter the non-executable prefix in
the Reader we pass to JsonReader. Instead, JsonReader's NON_EXECUTE_PREFIX and
Gson's JSON_NON_EXECUTABLE_PREFIX should be configurable to arbitrary Strings.
I didn't see any discussion in Issue 42[1], where this was requested, that
would preclude such a feature.

[1] https://code.google.com/p/google-gson/issues/detail?id=42
```

Original issue reported on code.google.com by `heath.bo...@gmail.com` on 17 Dec 2013 at 11:00