Improve README with a banner, download links, and improved wording.

- Add a banner image to the top of the file
- Use SVG badges
- Improve the description of the framework
- Add a Download section with links to release files and npm/Bower
- Streamline the Overview and Benefits sections
- Bring the Get Started and Get Help sections in line with the website
- Remove the Response Processing section in favor of the website version

Closes #216.