solve #306, and bin in $PATH make sense now

solve #306, and bin in $PATH make sense now  #196
Almost revert #173 which lead to the bug in #306

Also remove deprecated `customFds` option in spawn
Isn't it what [child_process.fork()](https://github.com/iojs/io.js/blob/v0.12/doc/api/child_process.markdown#child_processforkmodulepath-args-options) is for?