add get_file_descriptor
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
Ensure file is read in binary mode.