add protocol option (ssh or https) in config/settings.yml

The "example-app" breaks when the network (on which this app is deployed) can't use ssh port (22) for whatever reason, e.g. corporate FW setting. The error after "Create a commit" button is pressed may be as follows:

...
ssh: connect to host github.com port 22: Connection timed out
fatal: Could not read from remote repository.
...

This patch allows users to select https over ssh as git protocol if he/she likes. Also, when ssh is used, "The following SSH key was added to .." email message is coming whenever an app binds the service. It's a bit noisy in development.