Draft of the Generic IO protocol extension of the Simple Message protocol

As per title.

It is probably easier to read the document rendered, you may use [this](https://github.com/gavanderhoorn/rep-ros-i/blob/rep-xxxx_generic_io/rep-ixxxx.rst) url for that. I've also pushed a PDF rendered with [restview](https://mg.pov.lt/restview/): [link](https://github.com/gavanderhoorn/rep-ros-i/raw/pdf_attachment/rep-ixxxx.pdf).

Comments may be added in-line here (on the commit), or at the bottom of this PR.

Some points for discussion:

 1. should ``seq`` field of ros-i header be used instead of ``message_id``
 1. should copied ROS Packet header structure be shown in more detail in this document (seeing as the original source doesn't explicitly give more detail on used types fi)
 1. should message structures be defined using `shared_int` and `shared_real`, instead of current C typedefs
 1. should a `name` field be added to `IO_INFO` ranges
 1. repeat `timestamp` field in `IO_STREAM_PUB` for each published range, instead of one per msg
 1. should streaming support be required
 1. should explicit support for IO timed with motion be added (ie: synchronise IO write/read execution with `time_from_start` in trajectories), or should that be a responsibility of the client

The `type`, `start`, and `len` values uniquely determine the subscription.  These values will need to be used by the client to map to ROS topics (or some other semantically defined structure).  Would it be simpler to have the SUB call return an ID.  The same ID would be included in the PUB message.  This would ensure the mapping were correct.