Features from optional dependencies

mio implicit feature

Enables mio ^0.6.19

mio:

Lightweight non-blocking I/O

Affects ethcore-io::IoHandler.stream_hup, ethcore-io::IoHandler.stream_readable, ethcore-io::IoHandler.stream_writable, ethcore-io::IoHandler.register_stream, ethcore-io::IoHandler.update_stream, ethcore-io::IoHandler.deregister_stream