Simplify mail sender API to send synchronously by default

Right now there's only 1 API called send() which sends asynchronously (you need to call waitTillSent() if you wish to wait for the mail to be sent).

However the main use case is to send a mail synchronously (i.e. wait for it to be sent).

Thus it's simpler to modify the send() API to send synchronously and introduce a new sendAsynchronously() API to send asynchronously. This saves one method call when sending synchronously.