Fixed send_mail

Removed unused variables and made it simpler.

Usage:
$espresso->send_mail("to@mail.com", "from@mail.com", "subject", "msg",
"from name");

Check if is sent;
Etc.:
if($espresso->send_mail("to@mail.com", "from@mail.com", "subject",
"msg", "from name") == 1) {
// mail sent
} else {
// error
}