enabling on-demand loading/unloading fails

[rhaschke]

To fix #37, I tried to enable on-demand loading/unloading. However, now classes cannot be instantiated:

terminate called after throwing an instance of 'pluginlib::CreateClassException'
  what():  MultiLibraryClassLoader: Could not create object of class type test_plugins::Foo as no factory exists for it. Make sure that the library exists and was explicitly loaded through MultiLibraryClassLoader::loadLibrary()
The program has unexpectedly finished.

[rhaschke]

Will be fixed with #39.