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
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
Merge branch 'master' of git://github.com/contiki-os/contiki into nanosec