Add whatsapps as a native option in the UIActivity view

What are your thoughts on adding some common apps, whatsapp in this case, as an option on the native share window. We've added the code from this repo https://github.com/jberlana/JBWhatsAppActivity to this plugin to achieve sharing with whatsapp in a more native way.

If it's of interest, I'll raise a PR on it.

![photo](https://cloud.githubusercontent.com/assets/583627/4080963/4a61616e-2ee3-11e4-8f73-c0ac9acdfcc2.PNG)