Did you think Whastapp is only for your smartphone? Wrong! You can use it also on your RPi and it works quite well!
Yowsup is a python library developed for accessing Whatsapp without the original app (and on a machine that is not a smartphone, just like a RPi).
Unfortunately you still need a smartphone and a sim card for configuring Yowsup, but after the first steps you can leave it alone.