README: Modifying instructions - showHelp instead of showGear

The instructions were incorrectly referring to the API as showGear()
instead of showHelp().
Removing unnecessary <application> attributes and updating README

Updates to README:
- The <application> attaributes weren’t necessary. Removed instructions
for it.
- Made the Email gear more prominent in the intro section
- Mentioned that developers should choose 1 of the 4 gears
- Updated HappyFox & Zendesk example domains to something more generic
Change default HSApplication