Adding a new Power message

The ideal would probably be to add fields to Sense, but we can't do that until Indigo, now that there are Hydro firmwares in the wild.

Alternative would be to make breaking changes in an indigo-devel branch, and install them via source workspace on Hydro boats who want it, in advance of the full Indigo Kingfisher release.

@pmukherj, @rgariepy, thoughts on this?