Fix: don't hard-code Github username and password.

I stumbled on the username/password for the octomaps-app account in the code, and used it to login and create an octomaps application on Github. Changed the code to use the client id and the client secret, which gives you access to little to nothing, and externalized the configuration to a .yml file.

You should rotate the password for octomaps-app since it's in the code history.

Closes #25.