Update to markdown

- Spellchecking
- Update plain text to github markdown
Rename to md
Update README.md

Minor markdown => md renaming
Update of doxygen.to 1.8.1.2 using doxygen -u
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
Corrects some leftover uncapitalized printfs which may cause some platforms
to crash.
Removed undef
Merge pull request #216 from lebrush/printf-leftovers

Corrects some leftover uncapitalized printfs
Merge pull request #189 from sieben/markdown

Update to markdown
Merge pull request #197 from sieben/doxygen

Update of doxygen.to 1.8.1.2 using doxygen -u