Unit tests for adding and removing richlets

Because I couldn't find unit tests which test certain parts of Java code except ztl tests, I'd suggest to introduce basic support based on junit, EasyMock and PowerMock frameworks. Those frameworks can be replaced by the other ones if you tend to.

As ZK framework has different directory layout from default one (sources are in src/ folder, not in src/main/java/ ...), the unit tests are placed in test/ folder. Please tell me if it is not appropriate.

Please consider this pull request only after https://github.com/zkoss/zk/pull/59 because it contains its commits.