add debug statements to lifo
reset unused lifo slots
macro consistency and more DEBUG

addresses https://github.com/RIOT-OS/RIOT/pull/433#discussion_r8437084
add missing debug statement
make reset of freed slots optional

addresses https://github.com/RIOT-OS/RIOT/pull/433#discussion_r8472602
documentation, remove LIFO_DEBUG, break main

add documentation to lifo.h
remove LIFO_DEBUG and use DEVELHELP instead
make the main method which is included break
fix typo, improve doc
refine DEVELHELP section

more efficient to be better suited for ISR
made testcase more clear
Merge pull request #433 from LudwigOrtmann/lifo_debug

lifo debug and reset