Updated to mySQLi extension

Adapted the source code in order to use PHP mySQLi extension instead of
the old mySQL one.
Removed mySQL error message from message function

Removed mysqli_connect_error function call from error message creation,
it may expose sensible information in some situation.
Added error control operator

Added missing at operator before mysqli_insert_id function.
Merge pull request #6 from kirirur/master

Updated to mySQLi extension