Make ScheduleClose() idempotent

I think this fixes issue #441.

8b613d0 changed the `select` timeout to zero in the case where
NumCloseScheduled > 0. It seems that in certain circumstances (for
example, if `close_connection` is called in a connection's `unbind`)
NumCloseScheduled would get incremented twice, but only decremented
once, thus never again reaching zero for the lifetime of the reactor.

The user-observed effect was that one CPU core would be pegged at 100%,
because the zero `select` timeout caused us to go around the select loop
without any pause.

Changing it so that it is safe to ScheduleClose() multiple times seems
like the cleanest solution to the problem.
Merge pull request #445 from ept/double-close

Make ScheduleClose() idempotent
correct reactor_running? to return false after a process running a reactor forked
Merge pull request #455 from SamSaffron/master

Correct reactor_running? to return false after a process forks
add get_file_descriptor
Pause receive_data on-demand

When receiving large amounts of incoming data, we loop over recv() up to
10 times to drain the inbound buffers. This meant calling #pause from #receive_data
would previously not take effect until the next reactor tick.
Merge pull request #464 from eventmachine/pause-during-receive_data

Immediate pause from receive_data
update changelog
Add EventMachine.attach_server & EventMachine.attach_socket_server.
Simplify the attach_server stuff to 1 method, hold a reference to the socket.

Otherwise the FD could be closed by a GC.
Merge pull request #465 from benburkert/attach_server

Add EventMachine.attach_server & EventMachine.attach_socket_server
Return the signature ID from EventMachine.attach_server.
Merge pull request #466 from benburkert/attach_server_signature

Return the signature ID from EventMachine.attach_server.
Merge pull request #467 from brianmario/add-get-fd

Add get_file_descriptor which returns the FD number of a signature
Added concurrency validation
Added unit tests for EM::Iterator
Merge pull request #468 from morandanieli/iterator_fixes

Added concurrency validation to EM::Iterator
Add Code Climate badge to repo
Merge pull request #470 from mrb/patch-1

Add Code Climate badge to repo
Merge branch 'master' into ssl_certchainfile