Improved Unicode support and tests

- RLMProperty’s setterName is now correctly generated for UTF8 property names
- RLMQueryUtil now uses the standard RLMStringDataWithNSString function
- renamed UtilTests to UnicodeTests
- moved UTF8Object to RLMTestObjects to be shared with Swift tests
- ported the UnicodeTests to Swift
- improved test messages in UnicodeTests

Please note that because of rdar://17735684 ([openradar](http://openradar.appspot.com/radar?id=5222810825785344)), we might want to throw an exception if a user tries to define a model property containing UTF8 characters in the name.

Thoughts? @alazier @astigsen @kneth @rrrlasse