Cleanup
Support read and write events

Extend the driver message format to 8 bytes to support monitoring the
file descriptor for input and output events. The format is now:

    file desriptor : 4 bytes
    mode : 4 bytes

inert now may send 3 messages to the calling process:

    inert_read : fd is ready for reading
    inert_write : fd is ready for writing
    inert_error : error on fd
example: TCP echo server

Add an example of an echo server that doesn't use inet/gen_tcp.