AFAIK Moodstocks rocks!

VisionMobile / DeveloperEconomics
Their blogs and their reports. Definitely worth it.

Appannie provides great intelligence stuff / per domain, though quite expensive.
They bought Distimo which was the other source: http://www.distimo.com/appiq

In Localytics resources, you'll find interesting things: http://www.localytics.com/resources/

Hope it helps!

Sure, it's correlative for technical reasons, iTunes / AppStore does not provide (yet, might change) a mechanism for that, so 3rd parties have to build their own.
I recently discussed with people from Adjust and I was really impressed.
HasOffers / MobileAppTracking seems to be a good choice as well.

Unbounce is nice too for A/B testing your landing page.
There's one thing to remind regarding a "coming soon" app's landing page: it should become later the real landing page for your app, once your app is live. So if possible, the domain / url has to be the definitive one. If you do that, then you can start SEO stuff early on with your landing page (and maybe some PR preview stuff during your Beta), and this page will already be searchable when you launch your app + you won't have to do all the SEO from scratch.
Regarding fine tuning your app's landing page and the related text (with A/B tests, for example), there's also a strong benefit: real A/B test on appstores is not really possible, so it's very hard (and long) to find what converts well. If you do that stuff on your landing page and then your app page, you can optimize and hopefully find the proper assets (icon, screenshots, text, ...) to get a much better install conversion on appstores directly at app launch.

Hope it helps!
Alex J

Just looked at Flinto suggested by Dan, seems a great tool indeed.
I previously used Proto.io, which offers similar capabilities / user testing of a "fake but looking real" app.
Whatever, I highly recommend you to read http://svpg.com/flavors-of-prototypes/, because it just seems there's confusion in what you aim to do with your proto. Regarding the frameworks you mentioned, it seems you're looking to build a MVP / live data prototype, whereas most answers (mine included) gave advice for low-qual or a high quality user prototype.
Also, for the backend stuff, I advise you to have a glance at MBaaS (Mobile Backend As A Service) like Stackmob or Parse.

Hope it helps!
@AlexJubien

Well, the biggest cost here wouldn't be in the app, but in the server infrastructure. As usual with connected apps, people often forget they have to be connected to servers. The app itself is just a view to what the servers provide, so it's not the major cost. 20K $ to 100K $ should be enough.
On server stuff, I expect it to be a lot more, particularly if you aim for a wide audience (several million $).
On a Whatsapp clone, I'd consider:
- solutions for performing calls through servers, not that easy. Look at what Twilio provides. WebRTC is an upcoming protocol that will ease that (web & mobile).
- solutions for API, IM and user accounts management. Look at StackMob or Parse which are MBaaS (Mobile Backend as a Service).

Hope it helps!