Keep it secret. Keep it safe.

* Renames secrets.xml to secrets.xml.example and moves to project root.
* Generates secrets.xml file in generate-sources phase with real token values.
* Adds generated secrets.xml file to .gitignore file.
Add explicit generate-sources goal execution in circle.yml
Merge pull request #183 from mapzen/chuck/hide-secrets

Keep it secret. Keep it safe.