[TASK] updating composer dependencies

- removal of typo3/cms-composer-installers
- pinning dep versions
- switching to minimum-stability=dev (for cms-composer-installers)
- adding scripts (to remove typo3-cms-core installer symlinks)
[DOC] updating readme
[TASK] defaulting to 'dev' environment for build tasks
[TASK] updating phpunit xml
[TASK] cleaning up build file
[TASK] setting backend compressionLevel to 0; having problems with gzencode
[TASK] streamlining composer.json
[TASK] inclusion of composer autoload classmap; dump-autoload with -o option!