When using the `services.buttons` setting, the buttons specified in that setting get printed twice. This is due to the `services.buttons` variable being used twice, on [line 43](https://github.com/docpad/docpad-plugin-services/blob/master/src/services.plugin.coffee#L43) and [line 76](https://github.com/docpad/docpad-plugin-services/blob/master/src/services.plugin.coffee#L76).
