Merge pull request #289 from authmillenon/fix_sixlowpan_icmp_h

Fix sixlowpan/icmp.h
Start POSIX net [pnet] module
fd: initial code
Make fd.h publicly available

Needed for pnet
Generalize and simplify fd.h
Implementation of close()
Define stdin, stdout and stderr
Wrap RIOT socket API
Add auxilary headers