rename interface URIActionHandler to better fitting URIPathSegmentActionMapper
rename addHandler() method to match new URI path segment action mapper naming scheme
Refactor management of action command object for the URIPatchSegmentActionMappers

- replace root action command of DispatchingURIPatchSegmentActionMapper with optional command which is returned if sub-mapper of next URI token is missing
- partially replace term 'handler' with mapper in documentation and variable names
- (formerly default) action command of AbstractURIPathSegmentActionMapper is now used as the main action command object of all URIPatchSegmentActionMappers
- remove action command constructor parameter of SimpleURIPathSegmentActionMapper as the super class's action command is used
- fix tests
refactor instance variable name according to Clean Code
partially replace term 'handler' with mapper in documentation and variable names