Improve "Hello World" sample

* Don't require the developer to modify the redirect_uri field of
  his app.  Instead, we put the redirect_uri right into the
  sample app itself, as a command line parameter.
* To avoid confusion, only have the string "YOUR API KEY HERE"
  appear in the source code in places where the developer actually
  needs to change it.
Merge pull request #26 from mmorearty/improve-hello-world

Improve "Hello World" sample