Use ev_signal to avoid async-unsafe functions (Thanks Markus)

Use ev_signal to avoid async-unsafe functions (Thanks Markus)

Functions such as fprintf() might be unsafe to use in a signal handler,
see http://stackoverflow.com/questions/3941271/#answer-3941563

By using ev_signal, libev will use a tiny signal handler which just
passes on the information and then calls (outside of the signal handler)
our callback function which can use fprintf() and other unsafe
functions.

fixes #803