add information about installing the Installer/Plugin global
cleaned up plugin class and divided installer into Installer/CoreInstaller and Installer/ModuleInstaller

removed original Installer
updated tests
refactoring CoreInstaller
moved constant SORT_PRIORITY_KEY to ProjectConfig
cleaned up CoreInstaller
always use Copy as Deploy Strategy for CoreInstaller
moved root package config operations to ProjectConfig
moved redeployProject method to CoreInstaller
updated FullStackTest to use the CoreInstaller and install magento/core:* instead of Downloading Magento
bugfixing
fixed default type in Installer Constructor
cleaned up install and update methods