shell: refactor default shell command handling

before, we actually duplicated code for handling of shell commands
supplied to shell_init and those hardcoded by MODULE_SHELL_COMMANDS.

This patch refactors shell.c find_handler and print_help so there's
no duplicated code.
convert into host byte order in UDP reception
make TCP in destiny optional
udp_packet_handler uses printf, stack must be bigger
fix copy&paste typo
moved stack from header to C file
hwtimer.h remove cruft, document, rearrange

remove unsused hwtimer_t0* declarations
document hwtimer_now
move hwtimer_now outside the internal section
initializes watchdog properly for MSB-A2

It the watchdog does not reset the WDMOD, BSL pin remains ignored.
include version string
another NTOHS was missing...
set received parent lifetime when creating a new parent
initializes RPL instances
printf is used in sixlowpan mac layer, stack needs to be adjusted
use local packet_length variable in sendto

Having a global packet_length variable doesn't seem to be the best
idea. To avoid trouble, a scope local variable should be preferred.
remove vtimer object before (re)scheduling it
check if preferred parent was set before comparing
only add branch to version string if not in master
fix native stdlib.h include

Fixes segfault in OSX introduced by 45ffdea
shell: fix compile warning and add some comments
Merge pull request #337 from LudwigOrtmann/hwtimer_cleanup

hwtimer.h remove cruft, document, rearrange