check thread feature
fix test case: poll can't use for file descriptor on most platform
add note
Merge branch 'thread'

some libuv functions requires threading feature.
you have to re-compile your PHPwith --enable-maintainer-zts if you want to use threading functions.

NOTE: thread feature runs new php vm another thread. current implementation doesn't copy current
executor globals. so you have to load bootstrap file (like composer's autoloader) manually.