Added DB-based implementation of model Items.
Added demo web app "Task Planner" with screens, pages and REST services.
Improved JSON parsing error handling.
Updated staging plugin version configuration.
Improved InMem db file inconsistency resolution.
Added support for keeping meta-data in the DB files.
Improved DB duplicate file inconsistency resolution based on meta-data timestamp.
Introduced Admin screen (with shutdown button), disabled app screens config.
Implemented graceful page reload on AJAX event when the context is lost.
Merge branch 'master' of https://github.com/rapidoid/rapidoid.git
Added missing U module dependency in the JSON module.
Separated out array utils from U into new class "Arr".
Introduced reusable combinations of common buttons.
Fixed database serialization: meta-data was missed.
Refactored logic for special app screens and marked the Admin screen special.