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.
Zipfiles moved to generate_json and other fixes from comments

Files are now zipped when the json is generated, rather than doing
that on every request.

All plugin folders are zipped irrespective of the number of files
they contain - this has been done so as too keep things simple.

The API endpoints have now been versioned, so endpoints are now
/api/v1/plugins/ and /api/v1/download/

In the API, rathen than writing the json to disk on every request,
it's now done after 50 downloads.
Merge pull request #9 from musicbrainz/api

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