done major refactoring and implement signal fd (not tested yet)

- implement signal fd (only for linux, because OSX kqueue have ident collision problem when EVFILT_READ/WRITE and EVFILT_SIGNAL is mixed. for instance, stdout/stderr and SIGHUP/SIGINT, 
I think it requires deep consideration if you don't want to lose performance in OSX)
- reorganize API namespace (see README.md)