Added SimpleSignal file to contrib
From http://www.testbit.eu/2013/cpp11-signal-system-performance/
Copy some changes from Alec Thomas
From https://github.com/alecthomas/entityx/blob/302bf251f95577ecef8c9662f95651ae2cc49cd7/entityx/3rdparty/simplesignal.h
Change NULL to nullptr
The auto test for SimpleSignal is now running on MinGW32.
Use -std=gnu++11 instead of -D__NO_MINGW_LFS
Use signals for monster melee