Http Server: Add mime types for ico and txt

In the review process of the http server, it was noted that other Mime
Types might be used.

Specifically ico and txt are poular and will be used when the server
will return pages as a web server.

Signed-off-by: Amnon Heiman <amnon@cloudius-systems.com>
Signed-off-by: Pekka Enberg <penberg@cloudius-systems.com>