Update Observable.isInternalImplementation, get rid of NullPointerException

NullPointerException has been encountered during my tests. It is because java.lang.Class.getPackage() may return null "... if no package information is available from the archive or codebase" (documented feature).
Merge pull request #302 from rgerasimen/patch-1

Update Observable.isInternalImplementation, get rid of NullPointerExcept...