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.
I also realized that we'll want the api to be versioned, so /api/ should become /api/1/ or /api/v1/. This is so that if we ever need to change (as in break) the api, we can create a new version for it without breaking all old versions of Picard.