Use a mounted ResourceReference instead of custom IRequestMapper.

It is more idiomatic Wicket way and MountMapper is deprecated in Wicket 7.x
Add comments to the test methods
Add a test for AmdModulesRegistry.

Usage of AmdModuleHeaderItem registers a module in the registry
Merge pull request #6 from l0rdn1kk0n/use-resource-instead-of-custom-mapper

Use a mounted ResourceReference instead of custom IRequestMapper.