Bcmxcp

Taking all defined meters in http://old.networkupstools.org/protocols/eaton/XCP_Meter_Map_021309.pdf and define constants for them. Comment for each meter constants if it has been mapped to a nut entity.
Add a number of meter map to nut entity mappings.
Improve handling of command return status, and replace magic numbers with constants in doing so.
Improve hanlding of ups.load.
I do not have hardware to test the newly mapped nut entity mappings, will ask on dev mailing list if someone has hardware to test