add incby(n,v) to ganglia scoreboard for incrementing by a number

also, change the type from int to unsigned because overflow of
signed integers is undefined. This should not effect existing
metrics for ganglia agents because they are reset-on-read.

See http://stackoverflow.com/questions/18195715
Merge pull request #137 from satterly/feature/libganglia-scoreboard

add incby(n,v) to ganglia scoreboard and change type to unsigned