Update Registration.php

Replace mysqli functions by PHP PDO functions in all database calls.

Replace all direct calls to "new mysqli(DB_HOST, DB_USER, DB_PASS, DB_NAME)" by the function "private function databaseConnection()"
This function check if database connection already opened, create connection if needed and return true or false (if false, error message is added to error string of the class)
Merge pull request #154 from devplanete/patch-4

Update Registration.php : PDO support