Replace twitter app with generic http-post-auth

Twitter removed http basic auth from the api in august 2010, but this
underlying code is a good example of doing http basic auth in contiki.

The app has been renamed, and some fixes applied to make it build
cleanly.

This fixes issue #207

I have not included an example in this pull, as there was no example before, but I have one locally if people think it should be added.

Here's a trace of it hitting my webserver:
==> /var/log/nginx/access.log <==
172.18.0.2 - karl [04/May/2013:12:13:21 +0000] "POST /statuses/update.json HTTP/1.1" 404 536 "-" "Contiki 2.x" "-"

==> /var/log/nginx/error.log <==
2013/05/04 12:13:24 [error] 803#0: *21 open() "/home/www/site/statuses/update.json" failed (2: No such file or directory), client: 172.18.0.2, server: _, request: "POST /statuses/update.json HTTP/1.1", host: "api.example.org"