Remove custom exception

Adding custom exceptions like "InvalidArgumentException" add no value over PHP's built-in exception classes. The existing standard library `InvalidArgumentException` provides the exact same semantic meaning. Being able to catch a Psr\Http\InvalidArgumentException doesn't make any more sense than catching a regular \InvalidArgumentException. Custom exceptions should only be added when absolutely needed.

For more information, see http://my.safaribooksonline.com/book/programming/java/9780137150021/exceptions/ch09lev1sec4