gemspec: set licenses= attribute

Old versions of RubyGems which could not handle this attribute
are likely no longer in use.
bump unicorn dependency to 4.6.2+ for Ruby 2.0.0

This ensures we're compatible with the latest stable
Ruby version.
tests: update checks for Ruby 2.0.0

EventMachine/NeverBlock currently do not build on Ruby 2.0.0
doc: add HACKING document

Thanks to Alex Wolfe for noticing.
xepoll_thread_*/client: EPOLLONESHOT implies EPOLLET

No need to specify both flags for epoll (confirmed by reading
fs/eventpoll.c in the Linux kernel).
epoll/*: remove user-space array as active queue

This prevents pathological starvation cases where the user-space
ready-list can be repopulated infinitely.  With EPOLLONESHOT,
epoll itself may be used for this task (at a slightly higher
cost) by enabling read/write checks, as the epoll ready-list
preserves event ordering when used with EPOLLONESHOT.
manpage: update middleware-related documentation

-N/--no-default-middleware needs a corresponding manpage entry.

Additionally, the Rack::Chunked/ContentLength middleware comment
is out-of-date as of unicorn v4.1.0