Add security exception for /job/*/build and /job/*/buildWithParameters

(Reopening mocleiri/github-oauth-plugin#35 here, as this appears to be the new canonical fork for the plugin...)

This pull request adds a configuration option to expose the `/build` and `/buildWithParameters` URLs for all jobs.

In order to trigger builds remotely (via "Build Triggers"), an external client needs to make a request to `/job/:job-name/build` (or `/job/:job-name/buildWithParameters`). This request is protected by a secret `token` parameter, so exposing the URL publicly should be minimally dangerous.