Added the API as a part of the main website (/api)

The errorhandler returns a json based error response when a 404 occurs
on a url beginning with /api

The api currently has two endpoints:

    /plugins/?id=<id>	lists the plugin matching the id.

			if no id is secified, all plugins are listed

    /download/?id=<id>	serves the plugin specified as a download
			attachment.

			if no plugin is specified, an error is raised

			if the plugin has multiple files, they are
			zipped and served.

The api responds in application/json for all output and errors.