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.
ha! I always used to wonder why everybody else's APIs have versions in the url. I'll add this in the next commit.