Composer package

I made some changes, which allow to install this Behavior using the composer package manager. Also, I have edited readme to make it valid for such method ( and changed package name) and created package at https://packagist.org/packages/asper/cakephp-enum-behavior. 

Please, contact me in case of accepting pull request, I will change repository URLs in Packagist config. 

Also, I ask to choose some OpenSource license like MIT or GPL to make this package free to use for any user.