tec: Provide a Minz_Mailer class (#2476)

* Add Minz_View::_path method (replace change_view)

The `_path` method is more powerful since it allows to choose the file
extension. It is also Minz_Request-agnostic, which is useful to reuse
the Minz_View class in other places.

`change_view` is now deprecated and a warning is logged if we use it.

* Provide a Minz_Mailer to send emails

It uses PHPMailer under the hood and only supports PHP >= 5.5

phpcs.xml

@@ -6,6 +6,7 @@
 	<exclude-pattern>./static</exclude-pattern>
 	<exclude-pattern>./vendor</exclude-pattern>
 	<exclude-pattern>./lib/SimplePie/</exclude-pattern>
+	<exclude-pattern>./lib/PHPMailer/</exclude-pattern>
 	<exclude-pattern>./lib/http-conditional.php</exclude-pattern>
 	<exclude-pattern>./lib/JSON.php</exclude-pattern>
 	<exclude-pattern>./lib/lib_phpQuery.php</exclude-pattern>