LogIn & SignUp Feature added.

* _config.php : Database connection added.
* core\css\home.css : style added for new features.
* header.php : link added to logo.
* index.php : Some modifications.
* core\js\validation.js : username & password regexp check.
* join.php : SignUp page added (NW).
* login.php : LogIn page added (NW).