assigned-ids: address final feedback on PR.

Updated document:

 - added references to msg class sources
 - added section on identifier allocation procedure
 - clarified request procedure
 - updated ROS REP header (more in-line with REP 1 [1])
 - reduced length of vendor ranges to 100 (down from 1000)

I've interpreted the 'Post History' field in the header as a "record
[of] the dates of when new versions of the REP are posted to [the
ROS-Industrial mailing list]".

[1] http://www.ros.org/reps/rep-0001.html