Add a CONTRIBUTING guide.

GitHub will show this when someone's contributing. Hopefully it will
lessen the number of pull requests.
Add a --web-server-port argument for rb-site.

This allows the user to configure the port that should be used in the
web server configuration. It defaults to 80, so nothing changes there,
but it should be easier to automate a configuration using a different
port (such as when deploying behind nginx/varnish) now.

This is a more advanced option, and is not exposed during the wizard
flow.
Fix references to cgit id=

id2= is actually for SHA-2 hashes, which standard git installations
do not provide.

See https://fedorahosted.org/fedora-infrastructure/ticket/3687 for
additional details.

Reviewed at http://reviews.reviewboard.org/r/3930/
Revert "Fix references to cgit id="

This reverts commit 5c55e7a7d962e3143b3a7e28b0098d0e7139d75c. It
appears that some repositories have problems with this change.