Fix SQL injection vulnerabilities KyraD Patches 2-4
https://github.com/KyraD/simpleinvoices/commit/9004b82cf4ebd4bb279199d3a921c11c086d9fb4
https://github.com/KyraD/simpleinvoices/commit/4bd4e3f44792e31ba2496b875fb4f3d138cc3c34
https://github.com/KyraD/simpleinvoices/commit/77a9fc8c71317b6bf9699baa48f8758d795fcdb4
[PDO_mysql vs. mysql_real_escape_string](http://stackoverflow.com/questions/1742066/why-is-pdo-better-for-escaping-mysql-queries-querystrings-than-mysql-real-escape)