Update README to use "composer require", fixes #211

 instead of manually editing composer.json
Update install.rst to using "composer require", refs #211
Tweak language, no need to reference composer.json
Default application commands moved to getApplicationCommands, refs #413
Remove directory check in Config::getMigrationPath
add AbstractCommand::verifyMigrationDirectory, use for test command
add --[s]kip-directory option to create, use verifyMigrationDirectory