Support only PHP 7 and later

[Joungkyun]

Beginning with mysqlAES version 2, only php 7 and later are supported.
This means that the minimum version of php is 7.0.0.
For use with PHP versions prior to PHP 7, use the mysqlAES-1 branch or mysqlAES 1.x version.

First, the following features will be implemented.

• Support namespace
• Support function/variable type declare