Build App / Website for Bulk social media marketing

In my Android phone, I have a long list of contacts who use Whatsapp messaging. Using Whatsapp\'s original app, I am restricted to a broadcast limit of 25 contacts each time, not to mention having to select each contact manually. My customer contacts are quite a few thousands. This app which I require will have to be able to: 1) read contacts in my contact address book and load/show only those who are also using Whatsapp program. A (multi select and select all) for those contacts who are found to be using Whatsapp. A text box to enter message (and if possible to attach files). A (send button) to finally send/broadcast the message to all the selected contacts in batches of 25 to be within the broadcast limit of Whatsapp. The messages will make use of Whatsapp\'s service. It can be a standalone program or a program that interface with Whatsapp\'s mobile apk to achieve this batch sending. I will prefer it to be apk so that I can install into my phone rather than web version, but as long as my goal is achieved, either is fine. Alternatively, it can be a website where I upload my phone numbers in CSV file and they get pushed to whatsapp server to be sent out.