SSL support

Make it possible to open SSL connections.

Example:

```ruby
reactor = Ione::Io::IoReactor.new

f = reactor.start.flat_map do
  reactor.connect('github.com', 443, ssl: true)
end

f = f.flat_map do |connection|
  promise = Ione::Promise.new
  connection.on_data do |data|
    promise.fulfill(data)
  end
  connection.write("HEAD / HTTP/1.1\r\nHost: github.com\r\n\r\n")
  promise.future
end

puts f.value
```

It's not possible to start SSL servers yet, but that will come.