updated extension documentation; added sections dependencies, versioning and authorization
reverted events
updated code style, added autoloading and prefix examples
updated dependencies and testing
added link to composer docs about autoloading
removed based on samdark's comment from https://github.com/yiisoft/yii2/pull/1485/files#r8293528
updated autoloading info
updated database, based on @cebe's comment https://github.com/yiisoft/yii2/pull/1485#discussion_r8410809
Merge pull request #1485 from schmunk42/docs-extensions

updated extension documentation