fix man page about -x & -d can't be used together

reported by Moritz Lipp
update changelog
set default build type to release

thanks to Dmitry Yakimenko to hint me on the issue.
rename debug flag to no-filter flag
fix #43 to implement posix_spawn* calls
refactoring exec anatomy
fix tests on OS X

thanks to Alexander Poluektov for his machine and time.
set release date