This solves issue #56. Apparently, a previous commit changed sceneElementActor and broke the setRenderer method, making that it always restored the initial renderer.
I've also added a test so this will not happen again :)

Note: I've implemented a "reflection-based" implementation of the equals method that may be interesting for other tests...
The whole ChangeRendererActionTest has been reworked!
- Now, it relies on Apache's EqualsBuilder to determine equality among renderers (common-lang has been daded to engine's pom so we can use it in the future)
- It does not use json anymore for testing
These images are not needed
Merge pull request #58 from jtorrente/issue56-changerenderer

This solves issue #56. Apparently, a previous commit changed sceneElemen...