Version 2.0.0-beta.2

Second beta release of Pico's second major release. This is a pre-release!

* [New] Improve release & build process and move most build tools to the new
        `picocms/ci-tools` repo, allowing them to be used by other projects
* [New] Add page tree; refer to the `Pico::buildPageTree()` method for more
        details; also see the `onPageTreeBuilt` event
* [Changed] Update dependencies: Twig 1.35
* [Changed] ! Improve `.htaccess` and deny access to all dot files by default
* [Changed] ! Throw a `RuntimeException` when non-native plugins are loaded,
            but Pico's `PicoDeprecated` plugin is not loaded
* [Changed] ! Change `AbstractPicoPlugin::$enabled`'s behavior: setting it to
            TRUE now leads to throwing a `RuntimeException` when the plugin's
            dependencies aren't fulfilled; use NULL to maintain old behavior
* [Changed] ! Force themes to use `.twig` as file extension for Twig templates
* [Changed] Improve PHP class docs
* [Changed] Various small improvements