Change mysql_real_escape string to $db->prepare_input

This should resolve issues like the below, which currently occur during the plugin setup process.
`[xx-Nov-2014 00:xx:xx UTC] PHP Warning:  mysql_real_escape_string(): Access denied for user 'xx'@'xxxxxx' (using password: xx) in /home/xxx/public_html/store/coinbasepp_callback.php on line 34`