Log errors on encoding/decoding with the key name.

Logging class and key names on decoding and encoding. So you get something helpful:

```
2014-02-13 10:37:16.847 otest[35710:303] +[NSString length]: unrecognized selector sent to class 0x7fff73f67ea0
2014-02-13 10:37:16.848 otest[35710:303] *** Caught exception encoding value for key "klass" on class TestClass: +[NSString length]: unrecognized selector sent to class 0x7fff73f67ea0
```

Closes #238.