Default search adapter, autoload, update readme

 - engine move to the separate file
 - remove module for custome SearchAdapter
 - changed description of definition custom SearchArapter
 - in Installation section add mention about include js in pipeline
 - fix example of SearchAdapter
 - add processing of alternative select2 parameters
 - change routes
 - 'require' replaced by 'autoload'
 - 'send' replace to 'public_send'
 - add default SearchAdapter
 - added mention about lightweight ajax select2 without SearchAdapter
 - changed example of SearchAdapter
 - fixed example of usage select2_autocompletes_path
 - added mention about initializing on 'cocoon' event