comment and field fixes per #61 and #62
START PART 2:  https://www.drupal.org/node/2301273
apply patch https://www.drupal.org/files/issues/menu-2256521-MenuLinkTree-review-do-not-test.patch
convert createLink to addDefinition
docs fixes suggested by dawehner
Fix t() to $this->t()
some dawehner fixes
reapply review #15
some doc fixes
Part 2: 2301723 Added namespace in MenuParentFormSelector.php
Part 2: 2301273 Clean up in MenuLinkTreeInterface.php
Part 2: 2301273 Removed extra paranthesis from MenuActiveTrailTest.php