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.
Merge pull request #209 from karlp/kill-twitter

Replace twitter app with generic http-post-auth
Merge remote-tracking branch 'upstream/master'