partially replace term 'handler' with mapper in documentation and variable names
Refactorings

- extract private method for readability
- rename parameter names
- add todos
add new main entry point for the action handling framework: URIActionMapperTree, unit tests for fluent builder API
add unit tests for URIActionMapperTreeBuilder, evolved that class
rename custom assert methods
unit test for URIActionMapperTree#getRootActionMapper
implement subtree() functionality
restructure builders for path segment action mappers such that only a combination 'pathSegment(...).on(...)' can be validly used in the builders' fluent API