Adding geographical clarifications to REP-0103.

This pull request adds some clarification to REP-0103 about how systems aware of their global location and orientation should represent that information in standard cartesian ROS messages.

As the manufacturer of many outdoor, GNSS-equipped ROS systems, @clearpathrobotics is strongly interested in having clarity around these issues, as it helps us deliver value and consistency to our users, helps our users understand how their systems are configured, and helps everyone achieve harmonious integration between different IMUs, GNSSes, and INSes, as well as higher level localization and planning systems.

I'd prefer to save this follow-on discussion for later, but the larger context here is to work toward an amendment or follow-on to REP-0105 which would clarify what the meanings of the `odom` and `map` frames are in a globally-aware system, and whether a new frame is required which represents an absolute reference.
I agree. Unfortunately, there are several wikipedia pages covering this ground, and it's not clear which ones should be linked here. A sampling:

http://en.wikipedia.org/wiki/Axes_conventions
http://en.wikipedia.org/wiki/North_east_down
http://en.wikipedia.org/wiki/Geodetic_datum

The "East north up" article just forwards to "Geodetic datum" one, where I think "Axes conventions" actually provides the best overview.