added support for the embeddable-build-status plugin

The embeddable-build-status plugin provides an embeddable status icon, which can be used on external sites (for example a GitHub README.md). However, to allow this the github-oauth-plugin needs to allow requests made to this icon.

A new configuration checkbox, help file and business logic has been added to allow such an exception.

See the readme for the [grails-uploadr plugin](https://github.com/4np/grails-uploadr#build-status) for a demonstration.