Revamp landing page

The landing page could use more content. This will help improve search engine rankings in the long run. Other modifications will be made, as identified in #6 (comment 43093)

Proposed structure:

First section:
    - "Secure messaging, anywhere" tagline
    - "Peer-to-peer encrypted messaging and forums" tagline
    - Download link
    - Contribute link
Second section:
    - Explain Briar features
        Encrypted messaging
        Secure storage
        Offline connection
        Free and open source
    - Links:
        How Briar works
        Download Briar
Third section:
    - News link
Fourth section:
    - Who uses Briar?:
        Activists
        Journalists
        You
Fifth section:
    Get Briar:
        Fdroid
        Google Play
        Direct .apk download