actually use msg_queue in ip.c

msg_queue is defined but never used, hook it up so IP packets get queued instead of dropped when there is more than one.
change the name to ip_msg_queue to avoid naming conflicts.
also add msg_queue for UDP

All other layers in the network stack use a msg_queue to not drop messages, which in this context represent packages.
This finally fixes the random crashes when UDP network traffic is present. Turns out RIOT is not handling lost messages well.
add Makefile.tests_common

Contains common variables for all test Makefiles.
Include from test Makefiles.
remove USEMODULE comments and some whitespace
remove debug target from test_nativenet

It conflicts with an existing debug target
core: lifo: added the right author
Update libcoap
Merge pull request #740 from authmillenon/update-libcoap

pkg : libcoap : Update libcoap version
Merge pull request #680 from benpicco/fix_crash

set up message queues for the IP and UDP thread
Merge pull request #766 from mehlis/lifo-author

core: lifo: added heiko as author
Merge pull request #759 from LudwigOrtmann/tests_makefile

add Makefile.tests_common